The Tascam and Pandora are both good, you can save mp3s on the Tascam, slow them down and filter out the bass, and it also has some basic fx. The Pandora has a line in for an iPod (set the iPod EQ To 'bass reducer' if playing along with it!) you can also record 30sec loops and slow down / transpose etc, plus it's a full fledged multi fx unit with great sounds in it. And you can thread your bass strap through the clip!
For a cheaper no frills unit checkout the EA Earwig from the US.
Personally, I use a DI Box and iPod into a Behringer Micromon with great results, I've also just got a Pandora PX4D for dirt cheap for use outside of the bedroom, but haven't gotten around to learning how to use it properly yet!