Got this back form the local Jammer dealer.
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I believe that the Jammer Live should deal perfectly with the Conexant libraries, as they are General Midi compatible. The only problem you could encounter (with Jammer or any other MIDI program) would be an eventual latency problem (which I don't believe would happen, we all know that Nemesys internal drivers are well written... ;-). Anyway, this latency wouldn't be produced by the driving MIDI program (whatever it is), but by the response time of Nemesys' program, which, honestly, I doubt to ever happen. We did here numerous test with our programs remoting the GigaSampler, and we never noticed any latency anyway.
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We will test the GS/GM500/JL combination in a week or so.