Something to entertain is to go kit on first go. It's pretty much guarenteed success. Scratch has more potential of less desireable sound particularly on first effort. The last thing I'd want to do is dissuade someone from building from scratch - but it can be frustrating and patience is a requirement depending on complexity.
If you go scratch, check out headwize - think super cheap for low risk and forget about the rediculous "best". Personally I have not had great success with OTL amps and, for me, things started to sound much better and less finicky with transformer coupled amps.
It does not get any simpler than http://www.audiodesignguide.com/NewHeadphoneAmp/index.html - my amp is slightly more complicated (http://diy.ecpaudio.com/2008/03/SSS.html) but I have not looked back in years. You don't have to use these tubes, Lundahls are on the expensive side, and an alps pot is "perfect". Check out Edcor - super cheap, sound very good, and can be upgraded to Lundahl, Electra-print, Sowter, on and on, at any time. I don't see an impedance match to HD650's but maybe they can easily make a 5K:300 ohm, 5W, GXSE series for you - a friend of mine used them and they are very fine. So not as good as my Lundahl 1660's but I can listen to them all day with no complaints.
For tubes, look for high mu (25-80) and low plate impedance (1-2K ohm). Triodes like 417a (5842) and 6C45Pi and pentodes wired in triode mode like 7788 (E810F), 6688 (E180F), D3a, C3g, or E55L. At a min, read the Norman Crowhurst Audio Classroom papers.
Build it on a peice of plywood - one channel first. If you like the results move on to second channel. If you like the amp, then build an enclosure (the hard work). Don't forget to use free CAT5 strands of solid wire.
For power supply, I don't care what folks who have not tried this think - stack switched mode power supplies (SMPS) in series. I use 48V, 390ma, Cisco supplies from ebay. $5-$8 each - you need 4 for the amp I referenced. Takes 30min to wire up and it's dead quiet, super low imdedance, fast/detailed sound.
Finaly, never take a measurement with your hands in the amp. Turn amp off, clip meter leads where you wish, remove hands from leads, turn amp on, read meter. Of course if you use a conventional power supply with big caps make sure they auto discharge with bleeder resistors.
Go for it!