Apparently I was a little too enthusiastic about them because as we were leaving some guy turned to Matt and said, "You need to buy her a stereo, pal." To which I believe Matt said something to the effect of, "Yeah, really."
Once it sank in that this guy had basically been making fun of us (well, me), Matt proceeded to spend the next two days revisiting this incident and crafting a more clever response. These gems ranged from "I'm not your PAL" to various puns related to musical equipment. The last version that I heard involved lighter fluid and physical violence. I tried to tell him that his actual response was fine in that anything more involved would have made me think he was CRAZY, but he was undaunted in his pursuit of the perfect airtight comeback.
Personally, I think great comebacks are overrated. You always remember the bad ones, anyway. For example, one time in high school I told my mom she was stupid, and she fired back with, "I'll stupid your head." Another favorite of mine was the time that Dave's dad yelled at him, "You're trying to fuck with fire, but fire don't fuck!"
Lesson learned: Next time some random stranger semi-insults you, instead of stewing for days about what you should have said, just tell them you'll [adjective] their head. Guaranteed they'll remember you forever.