Hi Glenn and welcome,

I use BiaB MIDI almost exclusively. The GM MIDI sound source that comes with BiaB is at the bottom of the barrel when it comes to sound quality IMHO. A step up is the included sforzando. It uses the SFZ format. That is the same format as a number of entry level MIDI sound sources like Garritan's and Aria products. These are good sounding instruments, plus some are free.

At the same level are Soundfonts (SF2). There are a number of free SF2s available as well as free VSTis to run them like the

TX16Wx: https://www.tx16wx.com/

The next and biggest step up to great sounding MIDI are expensive programs like Kontakt and SampleTank:

https://www.native-instruments.com/en/products/komplete/samplers/kontakt-6/

https://www.ikmultimedia.com/products/sampletank3/

Both have sales occasionally so wait for one if you can or want.

However both have free players that you can use:

https://www.ikmultimedia.com/products/sampletankcs/

https://www.native-instruments.com/en/products/komplete/samplers/kontakt-6-player/free-download/

I use the full version of Kontakt. The free Kontakt player is only a player, thus you can not manipulate the sounds other than what GUI allows. Also one must be careful as some sounds are only for the full version, but there are a lot of sounds for the player. The full version will play both. I like Kontakt as there are a ton of third party sounds for it, some are free and some very expensive.

Another great sounding program is EastWest Play:

http://www.soundsonline.com/

This has great sounds but it is also very expensive, more so the the above mentioned programs.

The rule of thumb when it comes to MIDI is the more you spend the better the sound BUT if you have a good working of MIDI CCs, pitch bend, etc, you can get some very good sounds out of SF2s and SFZs. They are not as good as the others that I mentioned but they can be OK.

Good luck.

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