I'm using SampleTank 2.5 XL. It runs via the IAC driver (and can work with Windows as a DXi). It's not specifically a GM instrument, but it's easy to create and load a patch with any combination of instruments you want. It's a good option if 1) It's not out of your price range (good to buy it when IK Multimedia has a promotion going on), and 2) you have use for a high-quality sample player (not a full-featured sampler like Kontakt). It comes with a large range of good, usable sounds, and it's very easy to use. It integrates well with BIAB because it's got a kind of keyboard-workstation-paradigm: it's got your 16 slots, and you load a different sound into each one. Each slot responds to a different MIDI channel. You don't have to configure it that way, or open 16 instances of it to do this, that's just its basic setup. Very simple and convenient. There's also a lighter, less-expensive version, but I don't know what the limitations are. I think it might be a little expensive (or maybe not, depending on how you feel about it) to buy just to use with BIAB, but if it's something you would have a use or need for anyway, it integrates very smoothly with BIAB. Also, if you do need a full-featured sampler, you can probably open Bandstand instruments in Kontakt, correct?