hello,
as the HD800 seems to be such a hard to tame beast, i would definitely make every effort to hear them in as many set ups as you can.
meets, shows, retailers all come to mind.
this is rarefied territory. there are a lot of folks here who have spent many dollars, hours and (i would hazard a guess) agony getting the perfect system (for them) together for these cans.
as to upgrades, i would not even venture an opinion (which i am going to anyway), although the Schiit Valhalla 2 seems to be a solid and not outrageously expensive amp for them. although how well this amp would play with the K7xx is conjecture.
my reading here and elsewhere indicates that the HD800 require a commitment to a complete solution, dac, amp, going balanced etc.
the stop gap would be: Valhalla 2 (sell the Asgard), use your current dac (or upgrade to multibit, i dream of Bumby myself) and save for the HD800.
any pyrates who can chime in on the Valhalla 2 with K7xx would be most helpful.
we really need an amp comparison that would indicate the relative strength of amps that can "do it all." i.e. drive dynamics, planers, high z, low z, etc. etc.