Hydra vs PS Audio vs Equitech

Are you asking about power conditioners/regenerators, or are you asking about balanced power units (like Equitech)?

What is the difference?

They are totally different devices. A regenerator (I recommend the Exactpower EP-15A) reconstitutes the wall AC to produce a perfect 60 cycle sine wave at a a constant 120 V. In addition, most also provide EMI, RFI and Digital noise filtering along with (some) surge protection. This is the unit you should buy first.

Balanced power (best for front end stuff not necessary for amps) splits the AC potential in two. Instead of 120V between the the hot blade (of the plug) and the (nominally zero) neutral blade (plus your ground of course) it splits it so that there is +60 V from one blade to ground, and -60V from the other blade to ground. You still get 120V between the two plug blades, but the blades are always opposite polarity relative to ground. So any noise carried in the line (mostly hum but also RFI) is self-cancelling. Studios have used balanced power for years, but it was only UL approved for consumer use about 8 years ago.

All that said, if you are in a position to install dedicated AC lines for your system, that will give you a lot more bang for the buck. All you need then is surge protection, which IMO is best done at the breaker box.