I've owned the DR-6 and the DR-5 and a 250watt Threshold S/550-e. Both Classe pieces are beautiful and sound great with the 6 audibly a little cleaner, less noise, more detail, even more subterranean bass which is hard to believe - both are a little warm and dark versus tonally neutral. The DR-6 phono stage is to die for but the 5 isn't bad either. Although I never owned them together, either preamp would complement my Thresold which is a little light of neutral.
I've modified the DR-6 beyond anything anyone's seen here all Black Gates, custom $400 Shallco metal film 32 stepped volume control, new Power supply - the works. It's not quite in the same category as the best today, but very beautiful to listen to and you'd be very happy.
Not without a catch.... These amps are too difficult to modify due to the number of caps and circuit complexity and you'll ruin the beauty of their internal circuit layout - there's no room. Also, some of the parts have been well matched for complementary sound. The Black Beauty film volume pots sounded better than the "all out assault" custom job even though the latter was noticeably more transparent (Wish I had known that before I spent 8 hours building it).
You could find a DR-6 and upgrade the outboard PS caps and use Hexfreds only. Stop there @ 80%+ of all the way, and, instead of stock, you'd have a beauty of a preamp with no down side of wrecking a gorgeous classic piece. That's what I'd do.
After my experiment, I'm sworn to protect all Classes in original condition so watch out!
Oh, and whatever you do don't clean the faceplate with anything - the silk screening is very delicate.