Glad you are enjoying the 6xx! I purchased mine last year and really enjoy them.
There are lots of very well-measuring pieces of equipment out there at this price point depending on your priorities. Do you want a dac/amp combo, or a stack? Do you want particular connections, balanced inputs/headphone outputs? A remote and the ability to change output source so you can later run powered speakers from it?
If you want a very basic set of connections/functions, you could get a JDS atom+ dac/Amp stack for $210, and then use the extra money to buy a pair of Hifiman he400se (these are the other headphones I bought last year, and they are very different from the 6xx and which one I prefer depends on my mood/music choice that day.)
Explore our lineup of Element and Atom Amplifiers and DACs. Backed by a 2 year warranty and world-class customer support. Made in the USA.
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If you want something that looks nicer/is more robust re build (metal case and nice big knob) from JDS, you can get the Element II for $400. If you do, they will custom engrave it for you for free, which can look very cool and which I only realized too late! See
blog.jdslabs.com
On a very different end, there is the Topping DX3 Pro+ which is a combo dac/amp with a remote, bluetooth, but only a 3.5mm jack (won't affect sound, and the 6xx has both it and a 6.35mm adapter - the latter is more robust and more standard for larger headphones, while 3.5 is more often used with iem's) - total =$199
Amir has said that all of the above have no trouble with the HD650 (which is the same as 6xx) and his reviews tend to mention these headphones as he usually uses them to test headphone amplifiers. You can see reviews of these and other recommended equipment on this site using the review index. Click 'Review index->electronics and set your search/price parameters, etc.