These are not the same size of amplifier: FiiO AIR AMP has roughly 3x the power of Rotel A8. That is about 4.8dB, so it is a question of room size and how hard your speakers are to drive, not of one being the better amplifier.
| Area | Better | Why |
|---|
| Power | B | FiiO AIR AMP has more power - 90W against 30W into 8 ohms. That is about 4.8dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output increases modestly (40W and 120W). |
| Built-in DAC | B | Both decode on board, but only FiiO AIR AMP handles DSD. |
| Streaming | evenly matched | Only FiiO AIR AMP has Roon. Neither list is longer in a way that matters - check the one service you actually pay for. |
| Phono stage | depends | Rotel A8 has a built-in phono stage and FiiO AIR AMP does not. Decisive if you own a turntable, completely irrelevant if you do not - and an outboard phono stage is easy to add later either way. |
| Room to grow | B | Only FiiO AIR AMP offers preamp outputs and a subwoofer output - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Headphones | depends | Only Rotel A8 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Amplifier class | depends | Rotel A8 is Class A/B and FiiO AIR AMP is Class 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 | FiiO AIR AMP costs less - $300 against $499 at list. |
| Watts per dollar | B | FiiO AIR AMP is the cheaper watt - about $3 per watt against $17. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rotel A8 B FiiO AIR AMP
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?
FiiO AIR AMP is the easier recommendation: it leads on most of the measured areas and costs less. Take Rotel A8 if one of the "depends" rows above describes your system. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - FiiO AIR AMP has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 30 watts is already more than most people ever use, and the extra 60 watts would be headroom you pay for and never hear - we rate FiiO AIR AMP poorly for near-field use for exactly that reason. If you play records, Rotel A8 saves you buying a phono stage; if you do not, ignore that row entirely.
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.