Whatever you get, get tubes. Better for high impedance cans like the 650. Little Dot MK IV is nice, Woo Audio 3+ is very nice. If your budget is tight then a good place to start is this
Head-Direct.com | YUIN
The RCA Clear Top tube that comes with that amp happens to be a perfect match for his 650's. You could also show him this
https://www.tubeworld.com/index_high.htm
He would have tons of tube rolling options. The tube for this amp is the 12AU7, and there are lots of nice tubes on Ebay as well. I just bought a 7316 (military 12AU7) and it is magic. You could get him one on ebay or tell him to be on the look out for the 7316, its magic.
Here are some I have been stalking on Ebay, I want to accumulate some spares, he would be impressed if you gave him the amp and then said, "Hey Dad, I have this tube you should try"! It will sound better than the Clear Top.
http://electronics.shop.ebay.com/ite...=p3286.c0.m282http://www.head-fi.org/forums/f5/rev...1/#post4710473http://www.head-fi.org/forums/f5/rev...3/#post4913131