In this review, we will be comparing Gravis III and KUBE 8b, two Powered Subwoofer speakers from Sonus faber 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 Sonus faber Gravis III 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 Sonus faber Gravis IIIand KEF KUBE 8b have a single woofer setup however the Sonus faber Gravis III's woofer is 10-inch whereas the KEF KUBE 8b features a 8-inch woofer.
Frequency Response
Gravis III has a frequency range of 25-120 Hz whereas KUBE 8b has a frequency range of 34-140 Hz. With a minimum frequency of 25Hz, the Gravis III can go significantly deeper on the low side and provide stronger bass compared to 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
Both Sonus faber Gravis III and KEF KUBE 8b have Acoustic Suspension type cabinets. 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 Sonus faber Gravis III's and KEF KUBE 8b's external dimensions. Sonus faber Gravis III has external dimensions of 415.0 x 400.0 x 495.0mm (16-5/16 x 15-3/4 x 19-1/2inch) 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).
Sonus faber Gravis III is clearly the larger of the two speakers. Its body is 104mm wider, 121mm taller and 156mm deeper than KEF KUBE 8b.
Size, drawn to scale
Both speakers 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 surface and pushed back to the same wall, so the deeper one visibly sticks out further.
Sonus faber Gravis III needs about 97% more floor or shelf area. Sonus faber Gravis III is 156mm deeper, so it stands that much further out from the wall. Sonus faber Gravis III is 121mm taller.
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 Sonus faber Gravis III is approximately 1980.0cm2 / 306.9inch2 and base area of the KEF KUBE 8b is approximately 1003.4cm2 / 155.5inch2. The Gravis III requires 97% 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 Sonus faber Gravis III weighs 26.70kg / 58.7lbs and the KEF KUBE 8b weighs 11.36kg / 25lbs. This makes the Gravis III 135% heavier than the KUBE 8b.
Here is a comparison of how Gravis III and KUBE 8b's weights compare with the average, max and min weights in Powered Subwoofer class.
Weight
All Subwoofer Speakers
 Woofer Size |
10 inches
vs
8 inches
|
Larger woofer |
| Low Frequency Response |
25 Hz
vs
34 Hz
|
Generates deeper bass |
 High Frequency Response |
140 Hz
vs
120 Hz
|
Generates higher frequency |
 Height |
294mm
vs
415mm
|
Easier to fit in smaller spaces |
 Base Surface Area |
1003cm2
vs
1980cm2
|
Takes less floor space |
 Price per pair* |
600USD
vs
2700USD
|
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.
Sonus faber Gravis III needs about 97% more floor area. Sonus faber Gravis III stands 156mm further into the room. Sonus faber Gravis III is 121mm taller.
| General |
Sonus faber Gravis III |
KEF KUBE 8b |
| Brand |
Sonus faber |
KEF |
| Speaker Type |
Powered Subwoofer |
Powered Subwoofer |
| Max Amplifier Output |
600 watts |
300 watts |
| Frequency Response |
25-120 Hz |
34-140 Hz |
Amplifier |
|
|
| Watts RMS (Continuous) |
600-watt RMS |
300-watt RMS |
| Watts Peak (Max) |
900-watt RMS |
n/a |
Drivers |
|
|
| Woofer Quantity |
1 |
1 |
| Woofer Size |
10" |
8" |
| Woofer Composition |
Honeycomb sandwich |
|
| Tweeter Size |
- |
- |
| Aimable Tweeter |
No |
No |
| Midrange Driver |
No |
No |
| Crossover Frequency |
Continuously adjustable |
40-140 Hz |
| Firing Direction |
n/a |
front |
Physical Specs |
|
|
| Height |
415.0mm (16.3") |
294.0mm (11.6") |
| Width |
400.0mm (15.7") |
296.0mm (11.7") |
| Depth |
495.0mm (19.5") |
339.0mm (13.3") |
| Weight |
26.70kg (58.7lbs) |
11.36kg (25lbs) |
| Enclosure |
Acoustic Suspension |
Acoustic Suspension |
| Grille |
Yes |
n/a |
| Moisture Resistancy |
No |
No |
Connections |
|
|
Other |
|
|
| Parts Warranty |
|
2 Years |
| Labor Warranty |
|
2 Years |
Different price classes - Sonus faber Gravis III costs roughly 4.5x what KEF KUBE 8b does. Before paying it, read the note below about two smaller subwoofers.
| Area | Better | Why |
|---|
| Bass depth | A | Sonus faber Gravis III goes deeper - 25Hz against 34Hz. That gap covers the lowest notes of a five-string bass and a concert bass drum. |
| Amplifier power | A | Sonus faber Gravis III has the bigger amplifier - 600W against 300W RMS, about 3dB more on tap. In a large room that is headroom before the DSP starts limiting. |
| Driver area | A | Sonus faber Gravis III 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 | evenly matched | Both are sealed designs, so they roll off in much the same way. |
| Speaker-level inputs | A | Sonus faber Gravis III takes a speaker-level connection as well as line level. That is what lets it work with an integrated amplifier or a vintage receiver that has no subwoofer output at all. |
| Cabinet size | depends | KEF KUBE 8b is about 64% 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 $2,700, about 78% less. |
A Sonus faber Gravis III 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?
The usual trade-off: Sonus faber Gravis III measures better, KEF KUBE 8b costs about 78% less. In a normal room, placement will make a bigger difference than the gap between these two. Check your amplifier first: if it has no subwoofer or preamp output, Sonus faber Gravis III is the only one of these two you can connect at all. 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 $2,700 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.