Since it seems they did somewhat, even indirectly, ask for pointers, it's not like you're bludgeoning them over the head with unsolicited criticism.

Since they say they are impressed with your system, when discussing stereos or music and he again references your system, perhaps subtle suggestions like "have you tried turning down the treble a little", or some such might help. I would of course wait for the subject to come up naturally, though.

In either case, if they get defensive then I guess they weren't really that impressed with your system and were just being polite and I would just back off.