I don't think I'm quite clear on the use of the words "warm" and "dark" in the context of this thread.
When I use the word warm I'm talking about a rich, full sound that I associate with tube gear and analog front ends mostly, but to a lesser extent with Class A amplifiers. In the case of the tubes and Class A amps I think of this as due to the particular harmonics they tend to generate, and in the case of the front end I attribute it to the absence of the effects of digital sound reproduction and the presence of the effects of analog sound reproduction. Warm sound can quite pleasant but I think of it as a coloration caused by the equipment.
In this respect, I would definitely call the Pass Labs XP-30 neutral rather than warm (it's certainly not cold) and extremely transparent. I really don't hear any of what I would call warmth from it. What I do notice is that the XP-30 reveals quite a bit more of the subtle cues that indicate (or seem to) the size and characteristics of the room/studio/arena and the harmonics of the instruments and voices. This also tends to give a richer, fuller sound that can be pleasant (though not always) but I don't attribute this to a coloration but to the more revealing nature of the gear.
I own the XP-30 in combination with the XA160.5 amps. I haven't heard the XA160.8s yet but they will be in-home auditioned within the month. I would describe the sound from the XA160.5s as somewhat warm, reasonably transparent, and organic. When I use the term "dark" it's intended as the opposite of "bright", bright meaning an exaggeration of the treble range so dark then meaning a relative lack of treble (as opposed to heaviness, which is an exaggeration of the bass range). In this regard I consider my XA160.5s reasonably neutral. I'm not sayin' my perceptions are more accurate than anyone else's - just trying to define the terms being used and the way I use them for the sake of clarity. I guess my question here is, what is meant by those who are calling the XA160.8 "darker" than the XA 160.5? Less treble? More bass? Something else?
I agree with your definitions of 'warm' vs 'dark'.
I also agree that the XA160.5's are fairly neutral. Perhaps the XP-30 is as well but if so then the XP-10 is 'cooler'. The first time I auditioned the XP-30, I owned the XP-10 and Lamm M1.2 monoblocks. With the XP-30 going into the Lamms it was much warmer sounding than the XP-10, but the bass was also muddy sounding. I also auditioned the Ayre KX-R preamp at the same time and compared it to the two Pass preamps and the KX-R was very lean in comparison to the XP-30 and more like the XP-10 (although much better). This is the only reason I have the perception that the XP-30 is warm. I don't think it is dark as that implies rolled off treble.
As for how the XA160.8 sounds compared to the XA160.5, here is what I think after comparing the two today:
- There is more bass with the .8. It is deeper, punchier and thicker.
- The scale of the .8 is much bigger than the .5, especially at lower volumes. At about 90db the scale starts to get similar.
- It may be that the .8's don't have any less energy in the upper frequencies than the .5's and my impression is because of the extra warmth/bloom of the .8's (giving the impression of less treble). Today I noticed that by boosting the tweeter resistors on the Alexias by .5db it sounded more similar to the .5's.
- In my room, with my gear, the .5's sound more open on top and the presentation is leaner. This allows me to hear the texture of bass instruments more clearly. That said, switching from the .8's to the .5's the upper midrange of the .5's sound thinner and depending on the content I may miss the .8's.
- The .8's may have better 'deep' bass control than the .5's, but since I switched from MIT power cords to the Shunyatas I have no complaints in that area with either amp.
For rock music, I LOVE the .8's. The rich, meaty midrange presence and bloom makes it sound like a tube amp and is great for feeling like you're in a club. It makes me feel like I have much larger speakers and not want for Maxx's or XLF's.
For jazz, I prefer the .5's. I haven't listened to enough classical yet.
Both are great amps and I have a tough decision to make on Monday!