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 | A | JBL MA9100HP has more power - 140W against 105W into 8 ohms. That is about 1.2dB, which is a small step in loudness but real headroom on peaks. |
| Analogue inputs | B | Denon AVR-X3900H takes more analogue sources - 5 against 2. |
| Room to grow | B | 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 | JBL MA9100HP is D and Denon AVR-X3900H is AB. Class D runs cool and draws almost nothing at idle; Class A/B runs warm and is the long-established mainstream choice. Modern examples of both measure well - judge by the rest of the page. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | A | JBL MA9100HP is the cheaper watt - about $13 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 JBL MA9100HP B Denon AVR-X3900H
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?
Neither runs away with it - they trade advantages. Let the "depends" rows and the speakers you already own settle it.
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.