System phenomenon when playing loud

retaining coherence and detail as the music scales (get’s louder and bigger) is challenging. mostly (1) it’s the room, next most significant is (2) the set-up, and next (3) is the relationship between your amplifiers and your speakers.

what you describe is completely normal, and a proper step to finding higher performance. you are realizing that things should sound better, and interested in getting them to do so.

much can be done without spending money or changing gear. but to help we would need to know the room size, where the speakers are, what is on the walls (where are the windows?), where do you sit relative to the speakers, and how loud are you actually playing.

the gear you have listed should not likely be the big problem, if it’s a problem at all. it’s all competent stuff. it’s possible you have a part failing in one of your mono blocks too; do both channels react the same? i would do a little playing around only listening to one channel at a time to isolate this issue to a whole system problem, or an amplifier problem.

one other thing to check is if there are any adjustments on the amps for impedance or bridging or anything like that that possibly is getting tripped that changes the speaker loading.

This can happen very much so if the xover in the speakers are using iron core instead of air core inductors, they are cheaper but can saturate at high power and cause compressive effect of the dynamics, same goes for the driver voice coils when they get hot.

BTW: your amps JC1's are fine, they are not the problem.

Cheers George

Mike and George, thank you!  Excellent thoughts.  I don’t think it’s the room, but the room is about 15’ wide (speakers are along that wall) by 25’ deep, 9’ ceiling.  No treatments on the walls (yet), outside of normal pictures and other furniture.  It’s important to note that this high frequency suck-out happens in an instant.  Not gradual at all.  And it happens in both channels simultaneously.  Thanks for asking that question!  This leads me to believe that the problem may lay in the preamp or, as George mentioned, the speakers’ crossovers.  I would have never thought of the possibility of iron core inductors getting saturated.  Thanks George!  
In any case, I’m having a wonderful time testing the outer limits of this phenomenon.  It seems that, at least in this early exploration, that this happens, once, when the system has been running, at a good substantial volume (82 on the Line 1, and I do know this is an arbitrary, subjective number, given all the components) for a longer period (about 2 hours at least this evening), the phenomenon isn’t reoccurring.  
Spoke too soon, just happened again.  It occurs in both channels simultaneously, I think it’s got to be the preamp or the inductors, not the amps, given that they are mono blocks.
dare I start digging into the Triangle’s crossovers?  Not tonight.  Too many tunes to listen too!

thanks Dudes!

Dave

Looks like they are not opposed to using iron core chokes to save some money.
These could saturate when "cranked up loud" as you put it.

http://www.hifishock.org/galleries/speakers/triangle/_cache/altea-esw-2-triangle-280x196.jpg

http://www.hifishock.org/galleries/speakers/triangle/antal-ex-2-triangle.jpg

http://www.hifishock.org/galleries/speakers/triangle/altea-ex-2-triangle.jpg

If they were mine and I experienced the same problem, I would replace them with air core chokes of the same inductance and impedance, if you can find the same, you can get something a bit higher in inductance that’s close to the same impedance and de-wind it down to the right inductance. I did the same with some Yamaha NS-1000X or was it B&W801? long time ago, and the difference was very noticeable for the better when "cranked" up loud

Cheers George

My suggestion is try a different preamp. Or, your attenuator on the SF pre might be corroded a bit, causing the problem. If you can access it easily, you could try cleaning it with some contact cleaner/enhancer.

Hope you get it fixed, regards,
Dan