At the SAME volume, will a higher powered amp improve the sound quality?
No. It does not work that way. As the great Robert Harley said, "If the first watt isn’t any good, why would you want 200 more of them?" Good question. You have a great amp. You have great speakers. Unfortunately way too inefficient to sound good together at anything more than medium volume.
What nobody seems to want to admit, at 86dB you need 2 watts just to get to 89 dB. Doesn’t sound like much. No problem, you got 25. But remember, that’s measured at 1m. Do you sit at 1m? Not even. Sound disperses at a somewhat different rate for different speakers but the inverse square is a good first approximation. That is to say, twice as far away, 2 squared is 4, you get 1/4 the volume. Let’s say you sit real close, just 2m away. Now you need 2 watts just to get back to 86, which is not very loud. To play 89 you need 4 watts. At 92 dB its 8w. 95, now just barely getting to what someone might consider loud, you need 16, and your amp is just about tapped out. 105 is plenty loud, but 10dB calls for ten times the power, or 160 watts.
These are the most conservative possible numbers. Sit even a little further back, in a normal room with normal absorption, and using average volume not intermittent peaks, you can easily wind up with ten times that, and there are not a lot of 1,600 watt amps. Which to me sounds about right for playing those speakers at satisfyingly loud volume. No wonder your beautiful Pass is strained.
A huge amount of our perception of "strain" is tied into volume. With seriously inefficient speakers such as these its just real hard to get the mind around the fact all that power is going into.... not much. Just wasted. Not even going into heat. The speakers can handle much more. Its just a bad design, for anything other than moderate volume listening.
Which, remember again, first thing I said, you have great speakers and a great amp. Just not great if you want to listen at high volume. Horses for courses.