My impression of Pass Lab XP-10 (with respect to Conrad Johnson 17LS)

This is really not a review since my impression is not quite as thorough as a proper review.  I also want to compare the XP10 vs. the 17LS which has been my long time pre amp.   The xp10 is probably one of the best solid state preamp and the 17LS is one of the best tube preamp in their respectively price range.  The xp10 uses JFET which tends to have a more tubey kind of sound but they do a bit of a solid state haze.  The reason tubes sound more musical because it has mostly second order distortion (among other things) whereas solid state has more higher order distortion.  JFET is a square law device (meaning the output current is the square of the input voltage) which has mostly second order distortion just like tube, whereas bipolar transistors are exponential devices which have significantly higher order distortion.
 
First when I just got the XP10 it sounds a bit close-in and lacking soundstage.  To be honest I was a bit disappointed.  So I decided to let the preamp on for the entire week.  After that the sound opens up nicely but still the soundstage is not quite as open as the 17LS.  At the time I was using single ended cables and after reading some reviews online, people have commented that balance cables improve the sound.  So I bought a set of balance cables and now the xp10 soundstage is comparable to that of the 17LS, but there are differences.

Objectively, the xp10 has better soundstage definition, front and back, and better separation of instruments.  It has tighter bass and deeper as well.  It also has better dynamic and I think it is more linear as you increase the volume.  The 17LS after a certain volume level, it does get slightly distorted.  Many have commented that the xp10 midrange is liquid smooth which is true but I still think it's more like solid state liquid but not quite tube liquid.
But since being tube, I think the 17LS still has an edge on vocal.  It seems slightly more fleshed out and a touch sweeter.  On the 17LS, the vocal is almost like breathing into you.  There is a touch of distant on the xp10.  Also although its bass is not quite as deep and not as dynamic vs. the xp10, it seems to have a more palpable present and with slightly more texture.  And although the 17LS soundstage is not as clear as the xp10, being tube, there is a sense of present and live-ness that only tubes can provide.

In term of tonal balance, they are actually very close and interestingly enough, my 17LS with Electro Harmonix tubes actually sounds just a touch brighter.  But the xp10 is very smooth and the treble is very sweet.  One more thing, the xp10 is somewhat sensitive to temperature.  At cold, the bass is slightly thicken as if it's not able to open up.  But once fully warm up, it seems OK (although I am not sure it entirely goes away).

The xp10 is obviously a very good solid state preamp, I don't think there is any thing you can complain about.  But I am not sure as good as solid state nowaday, in term of sounding musical and having a sense of liveness, it can be equal to a good tube preamp.  I think a nice balance would be solid state amp, solid state digital source, and a good tube preamp.

Associate equipment:
Amp: Simaudio W7
DAC: Ayre qb-9 DSD
Cable: QED reference 40 and QED silver revelation
Speakers: DIY speakers using top of the line SEAS and Scanpeak and xover components.