Orchestral brass and fanfare - analysis of system

Lugnut, allow me to second your proposition.

Judit, I'm a tad confused & much intrigued. Let's assume for a moment that the recording is not at fault (i.e. it suffers from the usual d*nm compression!). If I'm not mistaken, the fact is that you detect a compression in the fundamental upper-mids that you happened to notice with passages of brass.

Then, if "mids and lows are good, however the highs become compressed at LOUD volumes. On low volume (...) the system relaxes into my comfort zone...", the prime suspect seems to be, electronics. I just can't tell if it's the pre, the amp or a combination of the above. I'm really tempted to join Stehno's suggestion -- but that me, not Judit (although I, too, judge by classical). Let's go further.

Brass is impolite in that its harmonics can go over 50kHz it seems, & well into subwoofer areas. So, as you note, the sub has helped a lot & a super-tweet (as per Mara, above) may help some more...

This said, the above still don't address the compression issue IMO, because this comes from the main speaker, not the peripherals.
Speaker: if they're incapable to take the drive from the amp, you'd be hearing driver distortion/blowing it -- which you aren't & it isn't. Remember, we're not discussing whether it sounds "correct" - just the compression.
Likewise with the cables: if they don't like the loads on either side, you'd be complaining about more than the specified instruments and, again, the issue is not how "nice" it sounds: just the compression. The wire would make a mess of it -- but wouldn't compress (unless badly connected -- and you'd hear that across most of the the spectrum).

So, there MUST be something off with the "engine" driving the speakers; if it were the pre, I think you'd be complaining about the lower register too. That leaves us with impedance & power capability in the amp-speaker combo. If the impedance were, say, to drop inconveniently at the fundamental of the brass, & the amp were tired, it would (& will) compress. OTO, I haven't lived with a KAV250 nor with 805's (but have w/801's), and don't know WHY it would find it appropriate to compress...

But, I put forth OUR answer to your question: can you borrow an amplification power-house, ANY, and try it out? Brass is difficult but, if it's anything BUT the amp you'll know just by trying another one.
THEN we can put forth intelligent propositions.

A long way of returning to Sten's idea -- sorry. Clink!

A final note (short this time). You may want to try a Symphonic Line Kraft 250 classA. It's one of the best machines I've heard for driving speakers full range (Nautilus). In your case the sub will make things even easier...