Power conditioning ( filtering ) is certainly possible. Alot of componets have some sort of L and C filter networks in their arsenal for the Mains power coming in. perhaps not so prevelant among audio componets but most computer power supplies and such have them built into the IEC plug. But these dont provide near the filtering capacity of something like the monster. Heres a pic of the inside just so you can see the difference between a standard power strip or surge protector.
Now, weither you WILL be able to heard a difference or see a difference is certainly debatable but is it POSSIBLE you will? Certainly, because it does really DO something unlike say, a pair of interconnects. But thats a different story in itself.
Heres an intersting link showing the difference between some cheap units and expensive ones. Considernig that it costs 30 or 40 bucks for a halfway decent surge protector 120 for one of these is pretty good deal. Noticed i said 120. The price went up! ouch!