Hello,
And thanks to all of you for your kind and helpful replies.
@Mulberry
Thanks for your amp recommendations. Your suggestion about the Norma is interesting — I’ve never listened to that brand before. I’ll keep it in mind if I end up moving away from the Pass Labs.
My only hesitation with the Pass is that it might be a bit too “high-end,” to reuse the expression mentioned earlier. It combines smoothness with precision, and it also has the big advantage of offering very fine volume adjustment (1 dB per step). The Nac202 wasn’t nearly as precise, and that was a problem for me since I listen at very, very low levels — sometimes the difference of just 1 dB can make or break listening comfort.
@Robinho
I agree with you on Class A. So far, aside from tube amps (a “no go” for me), only Class A amps have worked for me. I was tempted by the Sugden A21 and nearly went for it, but I read several reports saying it’s a rather lively amp with a strong sonic character. That feedback put me off a bit — though I’ve also read the opposite 
In reply to PeakMan
I was also surprised by the listening results compared to the session at my friend’s place a few months back. Of course the rooms are different (though with the same listening distance), but many other things change too: his speakers (PMC twenty5 23i), his speaker cables, his source cabling. I could only try the CD5XS as a transport since the DAC section wasn’t working. A shame, since that was what I was most interested in.
Of course the best option is to try gear at home, but it’s really difficult to get access to equipment (especially amps and speakers).
I did several tests this morning that turned out to be quite interesting.
I put all the stock power cords back on my gear.
For interconnects, I used an Acrolink 6N-A2400 II RCA (replacing my prototype cable which gives a huge open soundstage and a lot of transparency).
I think my setup should be able to work properly like this. The “fine tuning” can come afterwards.
First setup
Accuphase DP-60 (with its built-in DAC, 2x PCM58P-K) → Pass Labs → Vienna Haydn
In terms of rhythm and energy it’s fine. But there’s still this brightness that bothers me, especially on vocals or reverberant recordings (which are the worst for me).
Second setup
Accuphase DP-60 → Aqua La Voce (first version, 2x PCM1704)
The stage moves back, it loses some depth, I gain comfort, but on tracks with reverb the issue doesn’t disappear completely.
Third setup
Accuphase DP-60 → Audio Note DAC 2.1x (AD1865N + 2 Philips JAN 6DJ8 tubes)
I had previously ruled this one out because it added too much presence for me and had a high output gain. But maybe that extra gain is actually useful at my very low listening levels, to keep at least some dynamics.
This is the first time I’ve tried it in this setup with all stock cables.
Here, the brightness is completely gone and the listening comfort is immediate.
I was able to listen for two hours straight without any discomfort — and with real enjoyment, because it truly made beautiful music.
Of course, this is still fragile, and I’ll need several days to confirm. Maybe by tonight I’ll already have unplugged it because it doesn’t work out after all.
What’s clear:
- Changing the source has a huge impact on my system. I think the Pass lets everything through. If it ever becomes a problem, it will be because of that. But it also provides refinement, which I very much need.
- Power cords and interconnects also have a very strong influence. Power cords in particular can very quickly unbalance the system and lead to misleading conclusions in listening tests.
Thanks again to all of you for your help and contributions 
To be continued 