Both are AudioControl, from different ranges - the engineering approach is shared and the differences are mostly in power, connectivity and price.
| Area | Better | Why |
|---|
| Power | evenly matched | Effectively the same output - 100W against 100W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (200W and 200W). |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (95dB vs 96dB). |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Analogue inputs | evenly matched | The same number of line-level inputs (1). |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | AudioControl Rialto 600 takes up about 23% less shelf area (410cm² against 534cm²), and it is 57mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
A AudioControl Bijou 600 B AudioControl Rialto 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.
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.