I'm well aware that synergy is important. In the case of the fostex and k702s current is pretty critical though to making things sound good.
The bravo v2 and garage1217 are both class a hybrid offerings and trust me tube rolling didn't make the fostex or k702s sound good. I have a bunch of different tubes that I tried. It's that it doesn't deliver enough current.
I'd recommend one of the garage1217 offerings. If you want to go to the beyers or senns immediately I'd suggest the Ember if its within budget as its designed to have enough power for all sorts of headphones. If you plan for going for the low impedance cans the Sunrise II will do fine but it struggles a bit with high impedance headphones.
Then it's not a matter of lot's of power requirement or not, you just need a better overall amp than either the Sunrise or the Bravo/Indeed. In relation to a FR graph and the way impedence behaves, the K/Q70X doesn't need a lot of current and stays steady at the nominal rated z it is at. As I said, it's not really current i.e power that makes things sound good it's synergy.
If you're driving the HD600's or high z headphone, I'd look no further than a Bottlehead Crack which punches above it's price/sound performance ratio by a lot.
I'd look into a the Schiit line of amp's with that sort of budget, $350-400 should leave you plenty of left over $$ to get yourself a nice headphone with spare change.
You could Easily get a Little Dot MK IV and something like the Dt 880 600 ohm, the Ath W1000x, or the SennHesier HD 600 or 650. I'd recommend going spilt down the middle. $425 for cans $425 for an amp. And for $425 there are a lot of really nice options out there depending on what you want!
+1. Also a good recommendation.