Does Class A amplifier artificially smoothen sound

No, other classes artificially harden sound...;)

What DVD-A offers, at least in my system, is more detail. Positioning of instruments is more distinct as well. It's good in the precision area, and not so good in harmonizing and flowing instruments and voices together. In live performance everything is obviously more precise, but these sound sources propagate together toward you. It's nice to precisely know where the individual sources are in DVD-A. But if they approach you in a disjointed fashion, your brain would naturally do extra work to subliminally help you think that you actually listen to somebody singing in natural setting. That extra work is part of causing the headache. It's also interesting to note that the extra detail does not apply to the entire frequency range. Detail in the midrange,particularly voices, is actually reduced with DVD-A. Thirdly, the extra force and intensity of DVD-A sound is also unnatural. That extra pounding does not help the headache either.

My point is this sort of observation, with such descriptive words, occurs a lot in posting involving Class A vs other amplifiers, or tube versus SS. Well, this contrast between DVD-A and CD listening, at least with my system, appears not to support categorizing amplifiers as such.

Class A amplifiers don't exhibit switching distortion.
A Pure Class A amplifier is running 'full out' all the time--AB amplifiers run (about) 5 Watts of output all the time, and when the volume needs exceed that output the B transistors switch 'on' and that's where the so called switching distortion can be heard.
Put it this way. Let's say you own a speaker that's 88db 1W1M which is to say that a steady tone with at 88decibels will create a need for 1 watt of output from the amp. At 91db, you're using 2 watts, then at 94db 4 watts of power.
So, depending on room size listening tastes and program material, right at the edge of the sound exceeding 94 db, the B transistors 'cut in' and therein comes the 'switching distortion'. That distortion is more noticible in some amps than others, depending on too many factors to tend to in this short space.
The Class A isn't necessarily smoothening anything, it just doesn't have the issue of the swich from one bank of transistors to the secondary ones.
I hope this makes sense.
I am not the leading expert on this but know a little about it. If I can answer any questions on this just let me know.

Larry

Spatine, no arguments about the increased resolution of DVD-As! From your comments though, it sounds as if your DAC is creating HF noise which is making it painful. I doubt its the DVD-As myself. Its this comment:

In one case (redbook CD), I can listen to music for hours. In the other case using the same amp. and disc player, I got tired rather quickly (within 30 minutes).

that makes me think the amp has nothing to do with it in this case.

Atmasphere, I am using the analog out of the Meridian 808, thus its own DAC. Yet with such contrast between CD and DVD-A, this has nothing to do with the Pass XA60.5. Comments thus far seem to support my original thought that what you put in (meaning the source disc) is what you get out. Comments that Class A amplifiers are too relaxing, too sweet, or too pleasant might not be as definitive as some people think. In fact it's the same comments that many people think of all tube amplifiers. I don't think so either, but that is a different subject from the focus here.