AudioControl's Epicenter & the PG Bass Cube work in different ways.
The Epicenter is a bass restoration processor. It basically takes midbass and recreates it 1 octave lower. You will not get louder or have 'more bass' with already bass heavy songs with the Epicenter. The coolest thing with this piece is you don't have to use it in your car. If you don't mind re-recording your music, you can effectively use the Epicenter to 'remaster' old music by creating a loop with your PC with the Epicenter in the middle. This actually works remarkably well and I wouldn't be surprised if there's software out there that will do the same thing.
The PG Bass Cube is basically a low freq line driver; aka Bass Boost. This processor will give you more bass but it's also been known to be a woofer cooker. You'll have more bass with old music as you'll effectively be turning up the gains by increasing the voltage on low level signal into the amp. You'll also have a lot more bass with already bass heavy music and this is where the 'woofer cooker' rep comes from. Very easy to over do it.
Between the two, I prefer the AudioControl piece. It isn't pretty, but it does the job and I've never heard of an AudioControl piece breaking. If you have turn-on pop issues, use a relay for 12v switched.