Well...
...unless it's to the manufacturer's service center, I myself would never mail or ship any of my gear to an "unknown" entity...In fact several years ago a buddy and I drove from LINY to Moonachie, N.J. to hand deliver our troubled components directly to Pioneer...It was a nice day out and it saved the packing/shipping hassles.
I realize you aren't about to drive from Mass. to Illinois, but more often than not, sending things back from whence they came is the least troublesome way to go...and of course you did save all the original packing materials, didn't you? Some mfrs. can supply suitable materials...in most cases, bubble wrap and any old corrugated box isn't quite the way to go.
Most modern stuff requires wholesale parts replacement(entire PCBs and the like) it's no longer as simple as replacing the errant transistor or cap...unfortunately such work can be beyond the abilities of many service shops. Nowadays, without the proper documentation to work with, even isolating the source of the problem can be a daunting task. They may need service manuals, specialized software, etc. and it all costs...guess who get stuck with an inflated bill when all it's said and done...they aren't going to "eat" the expense, especially since they probably won't see your specific model ever again.
Then of course, even if diagnosed properly, they then must get the parts. They have to go to the manufacturer, so that just adds to the turn-around time...and markup.
I have a place I've used with some success for a number of years...they happen to be an authorized SC for some of my stuff, their track record is satisfactory but...the manufacturer can still perform tests/do work that may be required by service bulletins that are ignored or simply unknown to any and every hotshot with a soldering iron.
If I were you, I'd go to the local audio shops and ask around...good, reliable, honest shops get good word of mouth...perhaps one of the local enthusiasts has a place, perhaps he'll tell you you're just better off sending it back to Denon.
jimHJJ(...just my .02 worth of experience...)