Disappointment with Pass Labs - I'm looking for something else

I am a long time Pass fan. I bought a Threshold 250 watt amp in ~ 1980 A T2 preamp shortly thereafter, then a Pass x350 for 15 years. He does outstanding amp design and good preamps (not to my taste now). 
But from what you are saying about your tastes the Audio research preamp sounds like a great match. Your tastes sound like mine.  I owned a Audio Research REF 5SE for years, I now have a REF 6SE. The REF / Pass 350s is a good combination and would be better than the amp you have now.The speakers unfortunately are probably the real problem. The amp could be great depending on speakers. Not sure what your price range is, but you need some accurate high end speakers, B&W, Wilson, Sonus Faber, the choices are endless, I would stay mainstream then there are lots of reviews. Over the last 50 years I migrated from a mixture of components to All Audio Research Components and Sonus Faber Amati Traditional speakers, and an Aurender WE20se streamer. 

After reading my rambling post. I think my recommendation would be to get a set of high-end speakers... then assess your system sound. Put some serious effort into finding speakers to fit your musical taste. What music do you like? If it is punch dominated then B&W is a good choice. If you really like imaging maybe Wilson. If highly varied, with classical and acoustic then Sonos Faber...I recommend whole heartedly Olympica or higher.

So many recommendations

@opm, que genero de música escuchas normalmente, yo por ejemplo jazz, blues, rock, Clásico y eso, lo más difícil en mi opinión es reproducir el rock, Por la dinámica y porque las grabaciones originales son normalmente no son las mejores.
Con que alimentas la Bartok? Con CDs? O música en ficheros?

Yo tengo Klipsch P37F y otros con cornetas AudioKinesis Azels, y he tenido bastantes equipos de tubos y de estado sólido, quizás te puedo ayudar a que depures las respuestas.

While it is true that tubes sound good with Klipsch, so does many solid state amps as well. Midfi, the slightly.warm sound of B&K does the trick, Higher end SS Conrad Johnson solid state sounds delicious. Mcormack amps nice! Don't use the High Definition Audio Research SS amps, a waste of time and money, too bright!  I like the high sensitivity of Klipsch because it don't take a lot of power to get em going plus they are revealing of all the fine details and its like being in a front row seat at a concert.

Cornwall III's can sound dull, muted and warm with many amps/preamps. The new IV is much more alive, lively and sharp. I love my pass gear with Heresy IV and LaScala. Gorgeous, huge soundstage and sweetness that tames the highs of these speakers. The VIII's from Klipsch were much more dull sounding than what they make today. 

I have a set of XA60.8's and an XP-10 running Klipsch version IV's as well as Fleetwood Deville's. The sound is the best I have had in 35 years of HiFi. I know a pre-amp upgrade will improve it further as the XP-10 does lean slightly warm, Even so the sound is wide open and huge. 

The #1 ingredient to a great system is the room, then the speakers, then the pre-amp/amp, then the source/dac. Then cables. The kicker is....synergy. Add in something that doesn't have synergy when all connected together and it can sound awful. 

I wouldn't trade my Pass Labs for any other gear as I have not head anything better in my space, with my gear. But for others it may sound dull, lacking or even bright. All depends on the synergy. 

If you want a nice wide open soundstage, the Heresy IV will do much better with your pass gear than the CWIII. They can do big, sweet soundstage. They are not bright nor warm. They image like mad and sound stunning with Pass Labs. The 30.8 would sound gorgeous with them if they were set up right. Also, you have a world class DAC but it also leans warm. You have a warm amp, warm DAC and warm speakers. Yes, CWIII lean warm and muted. 

I believe if you changed your speakers you would be much happier. The Fleetwood Deville's are a huge step up even from my LaScala in every area. Even something like a set of KEF LS50 could bring you what you look for. Dynaudio bookshelf. B&W also can be fantastic. 

The gear you have far outclasses your speakers here. Even moving to Cornwall IV would shock you as they are much better than the III. Bottom line is that you have world class electronics. The synergy with the speakers is off.