I'm suspecting this is an ASIO problem or some config setting with your Fast Track USB. Do you have Biab set up to use ASIO or MME? If it's ASIO tell us what you have set in Biab's audio input/output window and the ASIO control panel. If Biab is set to use ASIO, change it to MME and see what happens.

Also, the TTS-1 is a good solid DXi softsynth, lots of us have it and it plays perfect with Biab. When you have the crashing, are you using only midi tracks using the TTS-1 or another synth? No Real Tracks or Real Drums or is it a combination of both? If you're not sure you can usually tell by the sound quality but also look at the instrument names accross the top, if any are green those are Real Tracks/Drums. Anything in yellow are midi tracks. I would try a test by picking a style that is all midi ie all the instruments in yellow, then trying an all RT style ie everything is green and do the same thing and see if it still crashes. If it does for one and not the other, there's a clue.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.