Om would be an alternative to jack-rack, less
straightforward maybe,
but much more flexible. Also with uptodate LASH support :)
http://om-synth.nongnu.org/
I have found ecasound to be a powerful alternative to jack-rack and even
ardour. Totally controlable by midi, you never have to watch to the
computer screen, quite useful when you are in live ;)
http://www.eca.cx/ecasound/
Regards
Philippe