Godfrey, it's not BIAB's fault at all, it's Coyote's fault!

I mean, since you are talking about the sound of MIDI, it has completely to do with the quality of your MIDI SOUNDS you use for MIDI tracks. Coyote is a middle-level solution for general purposes (as many other similar software), but if you want something REALLY good, you have to go to different solutions, such as the many specialized libraries for Kontakt.
For instance, for Celtic music:
http://www.timespace.com/product/CI-110/4/8888739/big_fish_audio_celtic_instruments_%28download%29.html
...but there are super-specialized libraries pratically for EACH music instrument you can imagine.
So, the procedure is:
- you make the MIDI tracks on BIAB, then
- for a better editing, you drop the obtained MIDI tracks in a nother DAW
- from inside the DAW, you choose a GOOD library (made of VST INSTRUMENTS) for each one of your MIDI tracks (fiddle, flute, harp, guitar...you decide...) made with BIAB.
I hope I understood myself....

Ciao
Alfredo