Bob, IMO you have two good options. TTS-1 is one of the better GM softsynths. The free soundfont player(s) and GM soundfonts that John also mentioned are as good as if not better than many of those purchased for $100 or less. One great features about soundfonts is that if there is one in your set that you don't like you can easily replace it. With a little work you can have things like 5 different trumpets, 3 different violins etc that you can use. There are a lot of options. Soundfonts are Ok but they are old technology, much like the GM VSTis and ST2 that is shipped with BiaB. They are usable but not the best in the world of MIDI.

But if you want really good MIDI sounds you have to purchase better sound sources and assign them. Assigning them is not a big issue but that is better done in a DAW like RealBand, Sonar etc IMHO.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware