Welcome!
I have and use the Epicenter. It absolutely does what it claims to do and the knob control allows you to fine tune the sub's output on each song if you want.
Some members have replaced the internal Bose amp with a more powerful one - reusing the OEM speakers and spare tire location.
Others, myself included, went to an aftermarket sub, amp and Epicenter as well. The combination of the three components give me all the bass I could ever use and more - I keep the gain on the amp and on the Epicenter at about 50%. The Epicenter bass adjustment is all the control I need, but the OEM equalizer still works in addition to the Epicenter knob.
It might be a good place to start - you'll get the most sound quality improvement when it comes to compressed music. For CDs or HD radio (uncompressed), the Epicenter doesn't provide as much benefit - but it does allow you to play more bass than the OEM controls alone allow.