No single best amplifier here. What decides it is what you have to drive, what you need to plug in, and how much of the system you want in one box.
| Area | Better | Why |
|---|
| Power | evenly matched | Effectively the same output - 105W against 125W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Analogue inputs | A | Denon AVR-X3900H takes more analogue sources - 5 against 2. |
| Room to grow | A | Only Denon AVR-X3900H offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Built-in sources | depends | Denon AVR-X3900H has a radio tuner built in. That is one less box if you still play discs or listen to radio, and dead weight if you do not. |
| Amplifier class | depends | Denon AVR-X3900H is AB and JBL MA7100HP is D. Class A/B runs warm and is the long-established mainstream choice; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Price | B | JBL MA7100HP costs less - $1,320 against $1,849 at list. |
| Watts per dollar | B | JBL MA7100HP is the cheaper watt - about $11 per watt against $18. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Denon AVR-X3900H B JBL MA7100HP
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 amplifiers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
Denon AVR-X3900H is ahead, but on only 1 of the measured areas, and it costs about 40% more. That is a lot of money for a narrow margin - JBL MA7100HP is the sensible buy unless a specific feature above decides it for you. Think about the whole system: the roughly $529 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
What a spec sheet cannot show: how either sounds with your speakers, how the controls feel day to day, and what the app is like to live with - which is where most all-in-one amplifiers are actually won or lost. Power figures are the manufacturers' own and are not measured to a common standard, so treat small differences as noise. If you can hear both, that will tell you more than any row above.