It sounds like you've got a great thing going, and YES, you can have a great HT with an RPTV. I recently got interested in HT when I saw a friends RPTV HT.
He had a Pioneer Elite with HDTV tuner, and a perfectly balanced 5.1 speaker system with separates. Amazing! I agree that the sound can add a LOT to the experience of envelopment, so your RPTV can be the centerpiece of an amazing system!
Technical Director, Home Theater Products