Have you tried tinkering with the thru settings in BiaB? This may help with the buffer overflow. It is not a common problem to my knowledge so some setting is likely causing it.
Turning off the thru on the Roland shouldn't cause it to stop making sound, just to not send MIDI out the out port. Are you *sure* you are hearing the SD-35, and not the softsynth in BiaB (via midi coming back from SD-35)? Is reroute MIDI to softsynth turned off in BiaB?


Make your sound your own!
.. I do not work here, but the benefits are still awesome