performance looping ?

[Tony Hughes <hughes.tony@gmail.com>]

I am a performance artist and have recently become interested in using
loops as ambient backgrounds for my performances.  I have looked
at the hardware (mainly boss rc-20xl and digitech jamman = based on
price range, under 500$ and performance capability).  I bought the
jamman and have been playing with it obsessively.  There is one
important thing that I want to do that I seem to be unable to complete.
  I want to be able to build a loop with multiple samples and then
unbuild it back to the original sample.  I can layer samples into
the loop, but cannot undo past the one undo that you are given.  I
can do this in a studio setting by creating loop one, copying it onto
loop two and then adding a sample overdub and copying to loop three
etc...but I would like to be able to do this in realtime - in the
studio setting I have to stop the loop playing in order to save and
resave to another loop.  Are there better ways to do what I am
talking about, or do I need to figure out how to work with what I
have.  

Thanks for any help. 

Tony