Paolo,
You're right, there's a lot of choices when it comes to an amp. In my head the first decision to make is whether or not you want the amp to be portable (pocket sized), transportable (easily moved from place to place) or home based (not easily moved). Next up is whether not you care if it's battery powered, able to use batts or AC, or AC only. Note that these first two decisions are tied together somewhat. Next on the list of hurdles would be budget which you might as well couple with the concept of whether or not you want a commercial amp (Ray Samuels, Gilmore, Headroom, etc.) or if you're comfortable with one of the smaller builders like MisterX, thrice, and others who make custom DIY amps. Some would argue that you get better bang for your buck with the custom DIY's since those people have no overhead - me personally I have no idea if this is the case or not and I'd guess that no one really does.
If I were to list amps that I'd buy for my Senns if I was shopping it'd go something like this:
PPA V2 - pairs very well with the Senn house sound by extracting a bit more detail than something like the M3. Cost = $300 - $400, non portable, debateably transportable (depending on version).
M3 - nearly as well matched with the Senns and better with other types of phones (Grado, AKG, Ety, Shure). Again, maybe transportable depending on config chosen. Cost = $250 - $400.
Gimore Lite - based on what I've read this would be a good amp for Senns. I have no personal experience with this amp. Cost (I think) = $300ish.
Shellbrook Ascent - again, just based on what I've read. Cost = $300.
There are literally a ton of other, those are just my choices based on the fact that I want a wall powered amp (changing batteries gets real old for me) and I don't care if I can bring it anywhere.
Just my 0.02,
Nate