Judging from spec from Audio-gd's website, the amp itself is fine (after all, it is very hard to make bad amp.)
Really, if you want to invest more on amplification for HD650, 600 and 800, you have to go balanced. And decent balanced amps are generally more expensive than most of super high-end headphones. For example, it cost me about 3k for my balanced DIY beta22 configuration. Woo audio one costs 2k, and Headroom is 1.7k, and I figure not much different for rest of companies, demanding at least more than 1k for balanced configuration. Also going balanced means that you will NEED balanced source, that means you need soundcard/external DAC that are capable of balanced output, yet another money to be spent for proper setup.
From my experience, one get much more for their money if he/she get better 'transducers' (headphones for our case) instead of amplification. There we have things like LCD-2, which I consider them as true improvement over HD650, and do not really require super powerful amplification to drive them well (very little difference when plugged my beta22 and my tiny portable player :P ) They cost almost exactly 1k, and they will probably give you the best performance for your situation.
In the end, my opinion is, saving more and going for better headphones is best choice for you.
Plus : And yes, one thing I can promise you for certain, is that DT880 indeed have more aggressive high compared to K701, but still tolerable (remind you I really hate pretty much all Grado headphones due to their high with exception of Joseph one and Alessandro). And stay away from SA5000 as far as possible.
Edited by wnmnkh - 9/13/11 at 2:16pm