Roughly 160mW and 7volts.
SEVEN VOLTS? That would be beyond excrutiatingly loud to listen to (and would probably pop the drivers).
InnerFidelity measured the 650, here:
http://www.innerfidelity.com/images/SennheiserHD650.pdf
Roughly .2 VRMS to hit 90 dB (which is relatively loud). Power requirements are fairly low too, as they're relatively sensitive cans - roughly .2mW for the same 90 dB.
To answer the original question: not a lot, most "headphone amplifiers" have orders of magnitude more power than you can ever realistically survive. So basically anything you get should have more than enough power.
So is that the end of it? Sort of but not quite. The HD 580/600/650/6XX family are all fairly reactive, which means frequency response has a relationship to output impedance of the amplifier. What does this mean in practical terms? Different amplifiers, if their output impedance is different, will genuinely and measurably sound different. This is the case with any reactive load (more resistive loads will exhibit this to significantly lesser degrees). Based on the Sennheisers' impedance we can say with certainty that higher Zout will lead to increases in midbass response, and you can see that modeled like so:
(where did this nice graph come from and what is it? this is actually based on the HD 650s, and comes out of a Russian-language review/measurement article, here: http://personalaudio.ru/raa/otchety/naushniki/sennheiser-hd-650/ - they did measure sensitivity and power requirements up near the top)
Whether or not you will like one variation or the other comes down to personal preference, and there is no "absolute truth" to be had here - they are reactive and always will be reactive, and Zout will interact with that and have an influence on frequency response. Your personal feelings about one configuration over another are all that matters. Personally I enjoyed the Senns with higher Zout and the slight boost to the mid-bass, but some folks will disagree with that notion. Generally I see tubes suggested for the Senns as "warming them up" or "improving the bass" and it makes complete sense - a lot of OTL tube amps tend to have higher output impedance. There are SS options as well, especially in the form of older/vintage receivers or amplifiers, but some newer boxes follow IEC convention and provide a 120R output (TEAC, Beyerdynamic, and Audio-Technica all have amplifiers like this).
I haven't used the Fiio mentioned, but I didn't have any major complaints about the E9 when I had it. The TI chip is a good platform. I don't know how similar or dissimilar it is to the E7 but my suspicion is that if the thing can provide at least 10 mW you're probably more than covered in terms of power requirements, so it just comes down to preference of how they interact with a given amplifier ("couldn't I just EQ it to taste?" yeah, you could do that too).
To the "dacs" question - that's a source component, it has no relationship to "powering" the headphone or anything of the sort. IME the differences between digital sources are quite small (if not nonexistent), so if whatever you have is quiet and clean and compatible with whatever you need to connect it to, I'd probably just stick with it.
Full disclosure: I'm not saying "high end gear isn't worth the time at all" or "all gear is the same" but I am saying that broadly speaking its very easy to hit diminishing returns with amplifiers, and even easier with DACs or other digital sources these days.
