System phenomenon when playing loud

Great Friday Greetings All,

I'm experiencing a phenomenon which I’ve never encountered before.  When cranking some tracks to an admittedly on the loud side level, sometimes the highs and mids become suddenly muted.  Not gone altogether, but very muted.  It occurs like a switch being flipped, no fade-out, it happens in an instant.  My system is:
Triangle Volante 260 speakers 
Parasound JC-1 mono block amps 
Sonic Frontiers Line 1 preamp
Recent purchases are a Panamax power conditioner and a Yamaha Cd-S2100 CD player.  Please note, I have experienced this phenomenon several times before acquiring the conditioner and the CD player, so it’s not those.  Can anyone guess if it’s the preamp, the amps, or the speakers that’s the culprit?
When I turn the volume down, the upper spectrum returns like nothing happened, and I can turn it back up some, but I’ve been shy about going back to the level it was at when the phenomenon occurred.  There are no attendant noises the happen with the suck-out, and there doesn’t appear to be any distortion when playing at that level, and I have listened for it. It’s not like the amps are clipping, audibly.  The tracks that this has occurred on are pretty intense rock tracks.

Puzzled, I think I’ll have a Martini.  Any thoughts appreciated,

Dave