One is a streaming all-in-one and the other is an amplifier. Judge them on how many boxes you want and what you already own, not on the spec sheet alone.
| Area | Better | Why |
|---|
| Power | B | AudioControl Bijou 600 has more power - 100W against 75W into 8 ohms. That is about 1.2dB, which is a small step in loudness but real headroom on peaks. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (150W and 200W). |
| Noise floor | A | Onkyo Muse Y-40 is the quieter design on paper - 100dB against 95dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | A | Only Onkyo Muse Y-40 has AirPlay 2 and Chromecast. |
| Phono stage | depends | Onkyo Muse Y-40 has a built-in phono stage and AudioControl Bijou 600 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 | evenly matched | The same expansion options on both. |
| Shelf space | depends | AudioControl Bijou 600 takes up about 56% less shelf area (534cm² against 1225cm²), and it is 104mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | AudioControl Bijou 600 costs less - $899 against $1,000 at list. |
| Watts per dollar | B | AudioControl Bijou 600 is the cheaper watt - about $9 per watt against $13. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo Muse Y-40 B AudioControl Bijou 600
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. If you play records, Onkyo Muse Y-40 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.