In this review, we will be comparing Radius 390 and KUBE 8b, two Powered Subwoofer speakers from Monitor Audio and KEF. Being Powered subwoofers, they both have built-in amplifiers so you don't need an external power amp or an integrated amp to power them. Let's have a brief look at the main features of Monitor Audio Radius 390 and KEF KUBE 8b first before getting into our more detailed comparison.
The sections below take each area in turn: driver and amplifier, frequency response, physical size and connectivity.
Drivers
The efficiency of a subwoofer is directly proportional to the total radiating are squared hence the woofer quantity and diameter.Both Monitor Audio Radius 390and KEF KUBE 8b have a single woofer setup however the Monitor Audio Radius 390's woofer is 10-inch whereas the KEF KUBE 8b features a 8-inch woofer.
Frequency Response
Radius 390 has a frequency range of 30-120 Hz whereas KUBE 8b has a frequency range of 34-140 Hz. With a minimum frequency of 30Hz, the Radius 390 can go slightly deeper then the KUBE 8b's min frequency of 34Hz.
Below graphs depict how these two speakers compare with the max, min and average values of the Min and Max Frequencies of other speakers in the Subwoofer class in our database.
Low Frequency
Subwoofer Speakers
High Frequency
Subwoofer Speakers
Cabinet Type
The Monitor Audio Radius 390 has a Passive Radiator type cabinet. Passive Radiator type subwoofers sports a non-powered cone that moves in response to the main woofer to move air out in order to generate stronger bass in a smaller body compared to ported or sealed designs. On the other hand the KEF KUBE 8b have Acoustic Suspension type enclosure. Acoustic Suspension aka Sealed type subwoofers have enclosures that are sealed hence no ports or holes exists on the cabinet. Sealed designs helps to provide more accurate and tight bass responses compared to ported designs.
Physical Specs
The size of a speaker can sometimes be an important decision factor due to space constraints or in some cases purely for esthetic reasons. In this section, we are going to compare Monitor Audio Radius 390's and KEF KUBE 8b's external dimensions. Monitor Audio Radius 390 has external dimensions of 348.0 x 331.0 x 385.0mm (13-11/16 x 13 x 15-1/8inch) whereas KEF KUBE 8b has external dimensions of 294.0 x 296.0 x 339.0mm (11-9/16 x 11-5/8 x 13-5/16inch).
Monitor Audio Radius 390 is clearly the larger of the two speakers. Its body is 35mm wider, 54mm taller and 46mm deeper than KEF KUBE 8b.
Below you can see the front view size comparison of Monitor Audio Radius 390 and KEF KUBE 8b in scale.
Comparison image of Monitor Audio Radius 390 and KEF KUBE 8b Size and External Dimensions
Base Surface Area Comparison
Base surface area of a loudspeaker may become a determining factor when the space in your room or desk is limited.
The base surface area of the Monitor Audio Radius 390 is approximately 1274.4cm2 / 197.5inch2 and base area of the KEF KUBE 8b is approximately 1003.4cm2 / 155.5inch2. The Radius 390 requires 27% more surface area than the KUBE 8b which gives it a small disadvantage on placement in tight spaces.
Weight Comparison The weight of a subwoofer may become an important factor if you need to move and reposition the unit often to find the optimal positioning, also known as 'Subwoofer Crawling'. The Monitor Audio Radius 390 weighs 13.77kg / 30.3lbs and the KEF KUBE 8b weighs 11.36kg / 25lbs. This makes the Radius 390 21% heavier than the KUBE 8b.
Here is a comparison of how Radius 390 and KUBE 8b's weights compare with the average, max and min weights in Powered Subwoofer class.
Weight
All Subwoofer Speakers
What's in the Box of Monitor Audio Radius 390?
Here are the items that are included inside the box of Radius 390:
- Powered sub
- 6-feet AC power cord
- 10.75-feet Mono minijack cable
- Owners Manual
- Important Safety Instructions
What's in the Box of KEF KUBE 8b?
Here are the items that come with the KUBE 8b:
- Powered subwoofer
- 6.5-feet AC power cord
- Speaker-level input block
- User Manual
- Important Safety Information
- Warranty Information
 Woofer Size |
10 inches
vs
8 inches
|
Larger woofer |
| Low Frequency Response |
30 Hz
vs
34 Hz
|
Generates deeper bass |
 High Frequency Response |
140 Hz
vs
120 Hz
|
Generates higher frequency |
 Height |
294mm
vs
348mm
|
Easier to fit in smaller spaces |
 Base Surface Area |
1003cm2
vs
1274cm2
|
Takes less floor space |
 Price per pair* |
600USD
vs
1150USD
|
costs less (*prices may change depending on the seller, location and time) |
Size, drawn to scale
Both subwoofers and both views share one scale, so everything here is directly comparable. Hover a box for its dimensions.
One scale for both, standing on the same floor and pushed back to the same wall, so the bigger one visibly takes more of the room.
Monitor Audio Radius 390 needs about 27% more floor area. Monitor Audio Radius 390 stands 46mm further into the room. Monitor Audio Radius 390 is 54mm taller.
| General |
Monitor Audio Radius 390 |
KEF KUBE 8b |
| Brand |
Monitor Audio |
KEF |
| Speaker Type |
Powered Subwoofer |
Powered Subwoofer |
| Max Amplifier Output |
220 watts |
300 watts |
| Frequency Response |
30-120 Hz |
34-140 Hz |
Amplifier |
|
|
| Watts RMS (Continuous) |
220-watt RMS |
300-watt RMS |
Drivers |
|
|
| Woofer Quantity |
1 |
1 |
| Woofer Size |
10" |
8" |
| Woofer Composition |
C-CAM |
|
| Tweeter Size |
- |
- |
| Aimable Tweeter |
No |
No |
| Midrange Driver |
No |
No |
| Crossover Frequency |
n/a |
40-140 Hz |
| Firing Direction |
side |
front |
Physical Specs |
|
|
| Height |
348.0mm (13.7") |
294.0mm (11.6") |
| Width |
331.0mm (13") |
296.0mm (11.7") |
| Depth |
385.0mm (15.2") |
339.0mm (13.3") |
| Weight |
13.77kg (30.3lbs) |
11.36kg (25lbs) |
| Enclosure |
Passive Radiator |
Acoustic Suspension |
| Moisture Resistancy |
No |
No |
Connections |
|
|
Other |
|
|
| Parts Warranty |
2 Years |
2 Years |
| Labor Warranty |
2 Years |
2 Years |
A passive radiator subwoofer against a sealed one. That is the most consequential line on this page: it decides how the bass rolls off, how it behaves in a small room, and how much output you get for the size of the box.
| Area | Better | Why |
|---|
| Bass depth | A | Monitor Audio Radius 390 goes deeper - 30Hz against 34Hz. A real difference, though most music has little content below 34Hz. |
| Amplifier power | B | KEF KUBE 8b has the bigger amplifier - 300W against 220W RMS, about 1.3dB more on tap. In a large room that is headroom before the DSP starts limiting. |
| Driver area | A | Monitor Audio Radius 390 moves more air: 10" against 8" is 1.56x the cone area, not the 1.25x the diameters suggest. More area means the same output with less excursion, and less distortion with it. |
| Enclosure | depends | Monitor Audio Radius 390 is passive radiator - a passive radiator behaves like a port without the pipe noise, and suits a slim cabinet; KEF KUBE 8b is sealed - a sealed box rolls off gently and is usually the easier one to integrate in a small or awkward room. Neither is better in the abstract; ported tends to suit films and sealed tends to suit music, and both generalisations have exceptions. |
| Driver placement | depends | Monitor Audio Radius 390 fires side and KEF KUBE 8b fires front. A down-firing driver is more forgiving of a grille-less look and of a hard floor; a front-firing one is the safer choice on a thick carpet or a suspended timber floor. |
| Cabinet size | depends | KEF KUBE 8b is about 33% smaller by volume. In a subwoofer that is a real trade rather than a preference: box volume is how you get output low down without a bigger amplifier. |
| Price | B | KEF KUBE 8b costs less - $600 against $1,150, about 48% less. |
A Monitor Audio Radius 390 B KEF KUBE 8b
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both subwoofers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
Monitor Audio Radius 390 is ahead, but on only 1 of the measured areas, and it costs about 92% more. KEF KUBE 8b is the sensible buy unless your room is large enough to need the extra. Films or music? If this is mainly a home cinema, Monitor Audio Radius 390 will hit harder for the money; if it is mainly for music in a normal-sized room, KEF KUBE 8b is usually the easier one to make sound right. Two small subwoofers beat one big one in most rooms - a pair evens out the room modes in a way no single box can, whatever its specs. At $600 each, two KEF KUBE 8b cost about $1,200 against $1,150 for one of the other, which is worth a thought.
One thing outweighs everything above: where the subwoofer stands. Room modes can add or remove 10dB or more at a given seat, which is far larger than any difference on this page. Before spending more, try the sub in two or three positions - the crawl test, where you put it in the listening seat and crawl the room to find where the bass sounds best, costs nothing and usually beats an upgrade.