One possible source of aggressive treble is poor amp/speaker matching, but this is not something which is obvious to most people. One would think good amp + good speaker = good results, but not necessarily.
If I understand correctly, the amplifier is a transformer-coupled tube amp. This kind of amp tends to deliver approximately the same amount of power into a fairly wide range of impedances.
On the other hand a solid state amp delivers increased wattage into an impedance dip, and decreased wattage into an impedance dip.
Let’s walk through an example:
So suppose we have an 8 ohm speaker with a 16-ohm impedance peak at 4 kHz. With a solid state amp, when the amp is delivering 1 watt into the 8 ohm nominal impedance, it is delivering 1/2 watt into that 16 ohm impedance peak. The designer probably assumed that a solid state amp would be used, so this speaker’s output is "flat" (or whatever the designer intended) with the amp delivering 1/2 watt into that 16 ohm peak.
Now let’s use a tube amp. The tube amp delivers approximately the same amount of power into that 16 ohm peak as into the nominal 8 ohm impedance. So at 4 kHz, the speaker is getting 1 watt rather than the 1/2 watt the designer intended. So we have a 3 dB peak at 4 kHz. That’s enough to transform "smooth treble" into "aggressive treble".
Rather than ditching your imo very nice Quicksilver amp(s), you might want to consider speaker compatibility with tubes. I can go into a bit more detail about it if you'd like.
Here is a paper written by a tube amp designer which goes into this from his perspective, mine being the perspective of a speaker designer:
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.htmlBest of luck in your quest, wherever it takes you.
Duke