I bought one of those a while back. If you like the sound of your O2 (I used to own one), I'd steer clear of this version of the Little Bear amp. I'm not sure it can power your AKG any more effectively, and it also has a somewhat grainy, very warm sound, and bass may be somewhat rolled off. It runs super hot. It only costs $40 for a reason.
I opted for the gold color. I'd imagine it would command a few bucks more on the used market than the other colors, but who knows. I'd just pick whichever version you like best. Subjectively, the glow of the tubes may "pop" more in contrast to the black chassis.
An amp might help a little bit, but it won't really make the headphone sound that different. The 440 is a fairly sensitive headphone and won't benefit much from the extra power an amp can provide. I just bought an SRH 440. I'll try it out with some amps and report back here.