I had a similar problem with my DR and here's my recent first step at solving it:
I emailed the guys at Eurotubes and ordered the hi-gain preamp (front end) kit on Friday afternoon.
First of all. My correspondance has been with a young guy named Eddie and let me tell you how easy going and informative he's been. Very good with me and making good recommendations and helping me solve me problem. Stoked.
Second of all. My tubes arrived on Monday afternoon (less then a business day later). 2 to 3 business days, my A$$. Prompt. I like that.
Ok, the important part is that I put the kit in (replacing a quintet of GT preamp tubes) and it was night and day. The hi-gain kit really tightened up my low end and my mutes sound tight, but not "sharp". So the amp still "sings" and "sparkles", it's just not flubby in the low end anymore. The hi-gain kit DID increase my gain response in a good way too. Overall a vast improvement that I'm very happy with.
Now, I'm running mine through a worn out marshall 1960a 4x12, so some of my flubby low end may be a product of the cabinet/speakers that I'm using. But, experience tells me that pre-amp tubes are everything in a Rectifier.
Step one = complete.
Next step will be a new cab. I don't know if I should be looking for a recto 4x12 or a standard 4x12...