I fitted a J-Retro in my Jazz... it's awesome, especially low end. Turned it into a pure bass machine.
Controls are complicated (compared to original setup!) but it's just fantastic!
It's very powerful though and there is a significant vol increase on switch-on, I've busted a couple of speakers in my Ashdown with it.
It's also very well made.
Can't comment on the Audere although a friend of mine has one and he couldn't get on with it at all.
It's fretless and just perfect for Drum&Bass!