Expert Pro 220 CI as power amp

[ogs]

The pass through function is useful only if you integrate it in a home theater system, allowing the 2 front channels to be driven by the Expert.
Otherwise, as said, it is pointless.

Now this is not entirely true.
The "Pass Thru" function is, as you say, primarily made for HT users. It disables the Devialet volume control and instead lets you set a fixed  level to match the output signal from the  HT amp (correct loudness from L & R relative to the other speakers in the surround set up) to have volume for all channels controlled by the HT amp.

The fixed level also works with a stereo preamp the same way as with a HT amp. Instead of having to use two volume controls (one at the preamp the other the Devialet) the volume is controlled only by the preamp.

In addition the line input has a sensitivity (gain) setting that should match the max output voltage of the preamp. The extra AD stage in the Devialet design will of course impact sound somewhat. ADH does all its clever stuff in the digital domain and a Devialet Expert amplifier will sound best with a good digital feed.