Piano and Trumpet can sound bright

I have the same cables (Kimber silver streak from CD to pre, PBJ from pre to power and 8TC speaker cables) ... the silver streak can be bright unless matched with a very high quality source. I have heard the same for the KCAG. I have a live room (hardwood floors, no heavy curtains, just an area rug, sofa and listening chair) so had the same problem with a brighter sound than I wanted. Moving the speakers further away from the sidewalls and toeing in them a little more to avoid the side wall reflection has solved the brightness. If that is not sufficient, try copper cables instead of silver. I am not familiar with your amp or speaker, but have heard very good comments on the Celestions as well as Quicksilver amps, so cannot provide any comments on these fronts.