I have iPod -->CLAS--->O2/Arrow (3G)/MiniWatt (v1)/M-stage/LF339/Sansui AU717/Toshiba SA7100/Pioneer SX1250....CMOY 2.2
So far I think the O2 is a no brainner if you are in the market for an amp. While I do not recommend tubes, if you already have a tube amp and you are into tube rolling, then this headphone is a must. Not because it's synergistic but because you can really tell the difference between the tubes. However, be warned that if you are annoyed by residual hum from heat build up or your tubes are in any way "noisy/microphonic" then you will not want to use these phones for extended listening.
Unlike the Grado's the Magnum likes voltage (similar to LCD-2 in fact) not necessarily huge watts, but voltage. Obviously the 1250 (165w) is not really a good match because even with the -20db engaged there is not allot of action on the volume pot. But surprisingly all the other amps pair well. In fact, I would say this is the most versatile driver I have encountered. As thelostMidrange has pointed out, these drivers (once burned in) pair well with just the bare source. But I find the CLAS-->O2 combo really synergistic, and its sort of portable (plus the O2 only costs $30 more than the drivers).
By the way my favorite portable set up is the nano--->LOD--->shadow, in fact that has to be my most used combo (because its so pocketable).
What it comes down too, IMO, is that these drivers are super revealing and as long as your source is good especially the files (!!!LOSSLESS!!!) you will be 90% of the way there. With something like the CLAS you are 99% of the way there. Choosing an amp then becomes a decision of what type of sound you are after, bright (nuforce, ibasso...) neutral (O2, Arrow, M-stage, 339..) or dark (RSA, Sansui, ALO...).
I hope this helps