Sean,
Regarding cables and racks, a few comments on what has worked in my system.
I dislike cones, bearings, elastomers and the like because of the inevitable sonic signatures, so I haven't put time into these and can't offer advice. The Denon does well on my recently acquired Grand Prix Monaco rack, which of course wasn't bought for the Denon. The GP btw is the quietest and least signature-prone rack I've had experience with. Previous to the GP, the best support was a Neuance alpha shelf, and for an affordable solution, I'd be tempted to try a newer Neuance replacement shelf with your existing rack. It doesn't need cones and should be evenhanded over a wide bandwidth, particularly if the bass is tighter and deeper than on the alpha.
On interconnects, my experience may parallel some of yours. Of the half dozen ic's I've tried, the Denon became unlistenable with cables known for forwardness or brightness, including a pair of Nordost Quattro Fil, and silver ic's with some residual silver brightness (e.g older Pure Notes). These were less problematic with my solid state cd front end cabled identically, so the Denon exacerbates the brightness. Interconnects that I've had good results with on the other hand are non-bright silvers (especially Empirical pc-holophonics) and MIT (copper,expensive) that provide neutrality, speed, detail, good staging, etc
Power cords may be system-dependent, but I've had a couple of dozen of them available during the Denon's residence and they all show the colorations they are known for. I agree with what I take to be the implied conclusions in Bill Gaw's review that numerous well-known pc's seemingly aimed at tubed systems sound variously thick and heavy, dark, slow, rolled off, fat, bright, whatever, with the Denon. Again, it responded best to neutral, quick, and detailed sound, and that correlated with low inductance, low capacitance, and generally unshielded cords. IMS, it shows a particular affinity for an MIT Oracle3 power cord; it also works with Pure Note and Empirical. It works particularly well with open airy sound (Pure Note Sigma, probably TG Audio) but the Sigma lacks bass IMS and is far from uncolored. For the record, my system is solid state and revealing, and the front end power cords plug into a Sound Application line stage plc.
A number of companies are working on mods for the 2900 - Exemplar Audio, Underwood, and Ric Schultz that I know of. Warranties are a lost cause after a year anyway, and things are evolving in the high rez business, so you may be trying something else a few years from now.