Harmon Kardon has been making car stereo since at least the early 80's (I use to sell it). It's not top of the line, but usually quite decent. I agree with reubet and M669326 as your first options, if that doesn't work out, I'd suggest upgrading the speakers, Polk, Infinity, many decent brands out there for upgrading. Upgrading the electronics nowadays is either impossible or difficult and expensive for most cars.
I also have XM Sirius in my MB and it sounds fine, not HD, but it rocks fine for road trips. Hope this helps.