I do not find them fatiguing in any way, nor "hot" on the high end; but then again, I am old enough that I do not hear anything above 10kHz.
I have the 2.9, Northpaw. I wrote about them above in this thread. I stopped using them for months because the treble was SO "hot". I guess I'm older because my hearing cuts off at about 8.5KHz, according to three of those online frequency sweeps. But I can definitely hear hot highs — not only tweeters, but the rising treble on moving-coil cartridges drives me nuts, and that's above 10KHz. I shouldn't hear it, but I do.
My tweeter is aluminum; is yours? I finally tried padding mine down with a 10 ohm resistor, and that helped a lot — they're my main speaker again.
I agree the bass is very good. My woofers are 10" so go a bit deeper, but only 2Hz acc'ding the specs. But I
don't like their location, inward-facing — and with that slanted baffle, I can't change it. The theory is good, it creates a "bass chamber" and they reinforce each other — but that chamber is only as wide as the speakers are apart, in my set-up 7–8 feet. That's too small when playing, for example, a full orchestra. The contrabass section is on the far right, and tympani + bass drum nearly far right, but the NHTs confine them to that 7–8 foot center — far too narrow a soundstage, if realism is the goal. I thought of trying passive bass-radiators on the outside of the speakers, but cutting into those beautiful cabinets would be criminal.
Here's what I tried, and it worked. I disabled the woofers; that left 3-way monitors with little bass below 100Hz (yours would be a 2-way). I put two small subwoofers, self-powered, to the outside. Their spec'd bass extension is the same, so no loss there. But the soundstage is now as big as Vienna. A
very satisfying change. If you still have the subs you mention, you might give it a try — "distributed bass" is all the rage these days...
I made another change. The 2.9s need 150W of good SS power to work fully — those woofers eat power. Even 100W don't work. So I tried bi-amping. With powered subs handling the bass, I used a smaller (70W) tube/Mosfet hybrid amp for the 3 smaller drivers. 70W is ample for them, and the sound is more to my liking, more musical, expressive, moving.
Anyway, speakers I admired but didn't really like, are now
so much better. Granted, your 2.5s are different, and my 2.9's transformation required two subs. But I thought I'd just share my experiment with you...