Grados aren't really that picky about amplification in general - there are plenty of "crowd favorites" from both camps, but the majority of tubed amps that are liked with Grados are hybrids (or at the bare minimum OTCs), just because of the low impedance nature of Grados.
In general, Musical Fidelity, Grado, Melos, and Mapletree amplifiers tend to be crowd pleasers, but honestly any reasonably quiet and well put together amplifier will be perfectly suitable (because again, they aren't really picky, and will sound good from more or less everything). I think if you've already got an amplifier that's quiet and clean enough (channel balance, quiet pot, etc) to handle your Denons, you should be perfectly set-up for whatever Grado you'd like to try.
I disagree. Grados are singularly well-damped. (See some of purrin's graphs and reports on Grado decay.) Connecting them to the current-FOTM super-low-output-impedance amplifers causes them to over-damp, resulting in the harshness and tizziness that many people cite for their dislike of Grados. The reason Grados do well with many tube amps is not necessarily the "warmth of tubes," but that the output impedance is quite a bit higher than your run-of-the-mill low-output-impedance SS amp. Grados do quite well on 15 ohm output impedance and higher.
This gets complicated to a certain extent because the low impedance of Grados causes fall-off with OTL amps as lowering the bass cut-off point results in extremely large and super-expensive output capacitors (which means shortcuts/cheaper alternatives are often used). Hybrids experience this fault, too, because many of their output buffers still require blocking caps and unless sized large enough, cause bass roll-off or phase distortion in low frequency response.
Bottom line, for the best sound with Grados - finding the right amplification is not necessarily so simple.