Line 6 Jam link.
http://www.youtube.com/watch?v=xIMszN0-B90
If this link sparks an interest check out a few more, it is crazy what you can do with this amp and a lot of it sounds like what you want to do.
It does only have 1 12" speaker, but it runs 75 watts through it and it gets dam loud. It also has an on board tuner. They run about $400.00 in the area I live.
This guy says it has a 24 minute record time, that is true, but you can save what ever you have recorded off to a SD card and have 24 minutes to play with again. It will record to a SD card in 3 or 4 different formats one being mp3. You can also bring what ever you have recorded back in from the SD card.
You can speed up or slow the tempo of anything recorded, you can change the key it plays the music in. So you could record yourself playing a blues rhythm let it keep looping and add a base line the second time through, then add piano the third time through. Stop save, now play it back in any key to see what it sounds like or to practice lead to. Heck you could hit record again and add a little lead to it, save it. Now play it back and add background lead to the first lead.
All the pre-recorded music on the amp was recorded in studios with live musicians playing. No computer generated sounds. I can feel the difference jamming with it.
There is a learning curve like anything else, but the book that comes with it is very good and it is a small curve.