Local tech just sent me home with the amp. Was beyond his expertise to troubleshoot or fix.
I don't think your amp is broken. Please allow me to share my experience:
I did not notice the noise when I bought my previously-owned DT50-112 because it was on the floor behind a desk in a large room (the seller met me at his work, which was an auto repair facility) and still had remaining warranty so I only wanted to make sure there were no obvious problems. The amp was so unused that the next day it smelled plus made all that noise and I freaked out! I brought it in for service, and I posted a hundred times in various threads here on the forum, searching for and sharing answers. Ultimately I bought a DT25 head, which amazes me daily in my basement. I'm hanging on to the DT50 for when I need more power, but may sell it some day if I find that I don't. Maybe. I'm predicting these DT's (especially the DT50 since it's no longer made) will be sought after and will command very high resale values in a few years. We're the lucky ones to have them now.
Here's the thing: Those DT50's are noisey beasts if you practice by yourself in a small room at low power. The noise floor is fixed depending on which voice you select, which is unfortunate and probably why they don't make them any more (which is VERY unfortunate). Turning up the volume makes absolutely no difference until you get to screaming loud, in which case the noise has long since become inaudible. When I did take the DT50 and play with a band, I never even noticed the noise. Not once, even between songs. I forgot all about it, actually. So what seems huge and horrible in my basement doesn't even exist under the conditions the amp was designed for.
If your goal is to play in a group, my advice is to ignore the noise when you practice because you won't hear it in a band. There was a guy who said he played in a church band and had to switch to voice 1 or standby between songs, but I think that's rare.
Here's what helps:
1) voice 1. Have you tried the soldano amps? (they call it "solo" something or other) there's clean, crunch, and overdrive models available, and you can get very nice hi gain tones in voice 1!!!!! If you don't have a POD HD, get one, or you can use MIDI to download the update for your amp, then download DT edit to set one of the voices (doesn't matter 1-4 or which channel) to one of these louder solos. Any amp using voice 1 will have minimum noise, but the solos seem to my ears to be the best high gain tones.
2) use 2 speakers. You've already got this covered. If you think your DT-212 is noisey, the 112 is actually a bit worse.
3) put a Mesa MBSPAX7 in the inverter slot. It's a very clean tube, and for me it took just enough of the edge off that with 2 speakers I could forget about the noise (except in voice 3, which is the worst).
4) be sure to try low power mode if you haven't already. Over time I found that I don't like it much, except in the case of the solo amps, which react very nicely with it in the DT50. If you do use MIDI and DT Edit, LPM can be turned off or on via MIDI commands and, from what I hear, probably saved to a patch in the POD HD as well! (if so, each patch would require a LPM setting as it won't automatically change states for you)
5) practice in a bigger room (if that's an option).
I hope this helps!
I wrote the above in response to your personal e-mail request, which did not state that you'd already played in a band and were concerned with recording. When I later saw your post, I copied that response here. Sorry for the TMI! Have you tried the low-impedance direct out jack for recording? (I have not, I just use my HD-500 direct to record.) Otherwise voice 1 and the SPAX7 may be helpful.