The first time I listened to a Denon headphone was at a meet in Calgary where someone brought a set of AH-D2000's. I tried them with a number of different amps and the impression I was left with was that they sounded a bit too muddy and closed-in. As soon as I paired them up with my Grace m903 everything changed. The m903 has a bit of a reputation for being either "bass light" or more specifically "lacking in bass weight"...something to that effect, but with the Denons they were able to tighten up the bass to a degree where it rivaled much more expensive headphones. Not only that, but everything else seemed to "click in" a bit better, especially the soundstage. The lowly D2000's sounded like a thousand bucks.
As a result of that meet I bought a set of AH-D7000's with J-money pads and combined with the m903 they are extremely enjoyable. I think that Grace must have designed the m903 to have an extremely tight low frequency presentation that is why they sound good with Denons and other cans that might otherwise be a bit too boomy or bloated. The flip side is that on cans that already have extremely tight, controlled bass the pairing can come off as anemic. That's the reason I have other amplification for my HD-800s and LCD-3s, and in those situations I use the Grace as DAC.
So for me the best amp for Denons is the Grace m903, but it comes at the price of worse pairing with some other desirable headphones. That being said, the m903 makes a good foil for a warmer tube amp and is an awesome, feature filled DAC regardless.