May I add a subwoofer to stereo system?

You certainly may, and please be my guest to do so;) Your Mezzo Utopias already go pretty low, so you will need to be sure not to go too far with the subwoofers. I've had excellent results with the REL Storm subwoofers in music systems. The Linn Sizmik is also good with music. For best results, place the sub(s) between the speakers. Do not put them in the corners where surround sound subs are placed, or soundstaging/imaging will be compromised. By the way, I received my Tri-Vista about three weeks ago, and it is an incredible player. Absolutely upgrade to it!