Thanks for the heads up on this! Still deciding what I'm going to do and am in no hurry, but this was helpful!
No worries. Glad you found it helpful.
Don't know if you saw my post, a few pages back, about playing the Hathor/Changeling stack at the outdoor charity event (and, earlier, in the too-loud bar, where I A/Bed the Handbox with a Quilter Bass Block) but, in short, I feel the head is a great match for the cabs tone- and feel-wise.
Given the power handling of the cabs (1000 Watts for the stack of two) and their moderate sensitivity, I wouldn't mind if the head (400 Watts @ 4 Ohms, guessing ~250 Watts @ 8 Ohms) were half-again as powerful. That said, if you're sharing your rig with people who might not use it as judiciously as you'd like, the cab-headroom isn't necessarily a bad thing.
Don't get me wrong -- I have and like kilowatt amps and have played them through multiple setups without issue, but I get nervous sharing a setup like that in certain situations. (Speaking of which, I might be wise to employ a CAUTION sign like you have,
@31 hz, on certain amps.)
If I were looking for a single cab to pair with an amp like the Handbox and cover a variety of stuff, I'd probably be looking at one of the 4 Ohm, 15" Hathor cabs (note: I've only played the twelves). The 212 is of course a possibility -- and the form-factor might suit some folks and situations better -- but, from what I gather from the AK site and conversation here, the 15 is at least as sensitive as two 12s (98 dB/W/m vs. 97), though the two twelves will hit harder (two motors) and handle more power if push comes to shove.