I'm not a major bluegrasser, but I've been working with Donna Ulisse recently, and we played a bluegrass festival in western Arkansas last weekend. There were some very good players (and singers) performing and it was interesting to see what the bassists were using. The sound man would let you use an amp for monitoring, but no DI signals to the house. Everyone got the same nondescript EV mic hung on this little gooseneck rig aiming directly on your G side F-hole. Actually, the stage and house sound was very good. As you'd expect, there were gut string players (myself included), but two standouts were the bassist for a group called The Bluegrass Brothers (from near Roanoke VA) who used a Kay bass with Innovation Rockabilly strings, and Mickey Harris with Rhonda Vincent who used Spirocores (didn't really see what his bass was) Mickey's Spiros sounded as fat and gut-like through the house as anything else. The guy with the Kay and Rockabillys sounded great too, he did some slapping and they sounded excellent for that. Among the rest, there was a mix of steel, gut and synthetic string users. Once again, it was a lesson in how the sound comes from the player, and how different strings can achieve sounds you might not typically expect.