One day later... Just boxed the M500 MKII to send it back.
At first I thaught everything was ok but nothing sounded interesting, not involving. So I put back on the RME ADI-2 PRO FS R and back was the midrange, soundstage and fun. OK perhaps the RME is an overkill in comparison to the M500 but than I swapped to the Topping D70s. The sound of the D70s is almost exactly the same as the RME. Slightly more recessed in the mids and perhaps slightly less deep bass but actually very "musical". Involving.
I told it was going to be subjective, so let's continue.
Because I could not believe the M500 MKII was so less enjoyable than both the RME and Topping, I also connected the Topping DX7s. Same results, both Topping DAC's and the RME soud very silimar and way more involving, transparent, musical than the SMSL.
Connecting the SMSL all fun was gone as was my interest in listening to the music.
All 4 DAC's were tested over USB in DAC-mode, on the same computer, every time connected balanced to the McIntosh C46 preamp.
I tested with the Neumark kh300 and with the same setup but the C46 RCA out connected to the power amp in of the McIntosh MA5300. Speakers are JBL 4412 studio monitors.
As musical it gets with any of the three DAC's, the SMSL was way back. Nothing wrong but ... nothing interesting.
So last night I took the lot downstairs and rented the ears of my wife. Downstairs we have a McIntosh C48 and MC252 connected to a pair of old fun JBL Ti5000.
Without connecting the SMSL I used our normal setup, using the built-in DAC of the McIntosh.
I sampled a track or three from Lee Ritenour and Dave Grusin (my wife likes this kind of music) and asked what she thaught. Nothing special, as usual - interesting music ;-)
Than I connected the SMSL - again XLR out to XLR in of the C48.
Same tracks.
Immedeatly she noticed a sweeter sound - guitar had less details. Somewhat recessed highs, lower mid. We listened a few more tracks than I had enough of it because we started talking and stopped listening to the music. I removed the SMSL and back to the original setup - immediatly music, fun etc... came back. Also : bass was much more detailed with slightly more bass and deeper.
Not wanting to spend any more time with the SMSL M500 MKII, back it went in its box, printed the label to send it back etc...
I do believe in measurements - that is why I bought the RME and the Topping products and why the M500 seemed a good idea as a second (third actually) headphone amp for on the go... - but I never could have believed that a product, tested with such good results would sound so dull.
I do not intend to ask for a replacement, I have had it with SMSL (this is the third SMSL product I tested - the M500 was better than the MKII - the SU-9 was fine-ish but both had the sereo-issue).
Back it went, I will stuck with Topping and be happy