You'd be surprised how many popular amps can't handle the HE-500's power demands. I think that the O2's power at 32 Ohms is perhaps marginal or just makes it. Many pure tube amps can't hack it. Many of the little dot's can only squeeze out 100mW at 32 Ohms. The Aune T1 can deliver 1000mW, however, at an output impedance of 30 Ohms the damping factor is not there so you can expect sloppy bass.
I don't think the Bottlehead Crack can cut it alone. I think you might be able to add on the S.E.X. kit. Besides not everyone is able to build this stuff and if you add up both kits and some other upgrades with assembly suddenly the price is not so attractive, you can probably get something else.
If one really wants a proper tube experience, I wouldn't recommend a hybrid amp. So for amps with tubes at the output, one has to choose wisely.
Edited by StanD - 1/8/14 at 11:05am