I like The BlueGrass with Banjo Solo style, but don't care for the bass in it. Can I use the bass out of Mountain Unplugged?

Yes, it's very easy. Right click Bass and in the popup> click Assign Real Tracks > enter Bass in filter by(bottom of popup) and chose the Bass you want then hit Generate Track. Normally you'll get faster responses if in Biab forum as this one is for new feature for Biab wishlist.

Thanks for your reply.. I meant to put it in the BIAB forum, Old age boo boo. I will put it into BIAB Forum.