I suggest you contact the Audiogon member "Samuel", who is Grant @ Shunyata Research. He should be able to tell you exactly what the difference are.
Now, with that being said, my battered brain seems to recall that the Hydra 2 was designed a bit differently than the other larger models. I seem to recall that the other models had more protection built in to them, (as the Hydra 2 was designed as a high current device that would handle larger amplifiers), and that the Hydra 2 did not have the larger bar type of buss. But again, that is based on my less than perfect memory from reading something a long time ago, in a galaxy far, far away. :-)
PS I use the Hydra 2 with my Lamm M2.1 monoblocks, and it works just fine. I will be honest and state that I don't notice much improvement, (probably due to the dedicated lines I have), but I also don't notice any reduced dynamics, which is what I worry most about when using power conditioners before power amplifiers.
