Just a SuggestionHi Apocalypse_HD650,
I have been in your shoes. It appears from your post, that now that you have a very nice set of headphones, you may want to do more with them than watching TV or playing on the computer. At least, that is what I infer, from your list of Amp candidates.
This is the route I took in order to learn what the "reviewers" were talking about when using terms like: warm, cool, bright, detailed, liquid, soundstage, depth, blackness, coloration, etc.. I purchased several flavors of the C-Moy type, a couple of Head Room types, a custom Pimeta and a couple of tube types.
I can now read a review and begin to understand what the reviewer is trying to say. I can relate, so to speak.
Here is what I have learned: the Quality of Source (CD Player, Sound Card, MP3 Player, etc.) + cable/interconnect + Hd. Phone Amp + Head Phone = Quality of Sound that reaches your Ears. Judging this is a personal thing that only you can do. Also, only you can determine how much "Quality and Expense" is really needed to meet your goals for the moment (remember - Welcome to Head-Fi...Sorry about your wallet.).
I would recommend spending as little of that $400 as possible, just now, on a Headphone Amp and using what you have left as a downpayment on a good audio source within your means. Having said that, I think you could get 2 amplifiers for about $175 that would allow you to evaluate, for yourself, some of the different sounds that are possible. At least you will have an idea of where you want to go as far the type of sound that appeals to you and some experience to make the equipment reviews more understandable. The two I have in mind could be resold later on at minimal loss to finance something more suited to your needs and tastes. I know some will chime in with other types and brands but that's good too.
Were I you, I would forget Tube Amps for the moment. I love them and have them, but you need to be more seriously into this before jumping into tubes. Also, tubes a a bit more delicate and expensive to maintain and tune.
You can pick up a C-Moy for around $50. Pereferably one with a single 132/134 OpAmp. It's portable and battery powered ( some come with the ability to connect to a "wall wart" transformer for AC operation). Battery life should be 30 to 40 hours. 470uF caps minimum, 1000uA will get you more thump to the bass. With Senn-650's it will work fine . For about $95, you can also get a Maxi-Moy (http://www.shellbrooklab.com/) or similiarly built amp. This gives you two better OpAmps ( usually 227's) , one per channel ( left and right), better circuitry and buffers and will sound quite different than the cheaper C-Moy. Your 650's will sound much improved. The Maxi-Moy is both battery and AC powered.
I think what you will find is that the C-Moy has a nice bass thump but sounds a little rough compared to the Maxi-Moy. the Maxi-Moy will just flat out blow away the C-Moy all over due to its two OpAmps and other components.
Now you can follow the discussions here on other types of amps and understand the terminology used to describe the various qualities of different amps. Then if you decide to try another type, sell these two and you've got your downpayment. In the mean time, take the $225 left of your $400 budget and begin to look toward upgading your Source.
There are other OpAmp chips to explore too that may be a better match to your ears and the 650's than the Burr Brown type I've mentioned. Search out Tangent's excellent articles on this and the differences between the C-Moy, Mint, Pimeta and PPA type amplifiers.
Any of the Amps mentioned in your post will be a major improvement should you decide to just jump in and use all your budget. My opinion of how to spend your money is just that - an opinion. Hell, if I had the $400,i'd pobably blow it and a few more bucks on that beautiful Shanling SCD-T200 Tube SACD Player I just lost a bid on at Audigon LOL!
- Good Luck - Tom