What's you opinion?

A good sub is one that when properly set up cannot be heard.

Why do audiophile's spend so much time and money on expensive amp stands? Why not just put your amp on top of your sub? The vibrations from the sub would have no effect on the sound would it? The answer must be no or they would not stick the amp inside a powerd sub. Also why do so many subs use cheap low-fi amps to power the bass when we spend thousands on our amps for the rest of our systems. Is the bass not as important as the mids and highs? Food for thought.

Biggest problem I see with most sub set-ups, right up there with improper placement, is trying to integrate a sub with small satellite speaker systems that have little or no bass below 80Hz. This forces people to push the subs up into the 100+Hz range where they really don't work as well. I started messing with biamped and passive subs back in the late 70's as LS3/5A's and KEF 101's and several other monitor speakers rose to popularity. The results were never completely satisfying. I finally gave up. Years later I read a quote that finally pointed me in the right direction. It roughly said that "It is difficult to integrate a sub with "main" speakers that have poor bass output". Recently, I have heard some really good main+sub set-ups where the mains were full range speakers with GOOD bass to about 45Hz; they just needed a little help in that last octave. Crossed over at about 60Hz, many of these set-ups sound quite good. (You still need correct placement.)

Well...if you think there are very few decent loudspeakers...try finding a musical sub...slim pickings...there is REL,Sunfire(much to hi enders dismay),M&K, hi end Velodyne...ect...

Subwoofer shouldn't sound like anything. Meaning if you hear it, it's probably set too high or wrong room placement.
HSU has a VTU-2 (?) about $500 or REL are both good.