This is probably the toughest thing to get right in a system-- silky treble without loss of detail, and treble bite (on brass & strings) without aggressiveness. Source, preamp, amp, speakers all make or break. I think one giveaway that it's not right is if a silky, pretty treble is accompanied by weak dynamics lower in the frequency range. In my tube system at least, I've found that steps taken to improve control & dynamics usually extend and refine treble. Since the amp has the key roll in dictating system dynamics, perhaps this is where your friend is coming from with his observation about its importance.