Far as I know you are correct in that gm.dls is the Windows default GM sound set.
In the old days we would swap other files in place of it to get Windows default to sound better. I *think* you can still do that if you have admin rights, since it is a Windows file. <grin>
But Windows may set it back at any point in time with an update frown

Conversion from a Soundfont (sf2) file to DLS was pretty simple. So using a Soundfont for your default windows sound set was possible if you were brave enough.
BTW, this is the same file Coyote WT would use as the default sound set, which is cool.

From memory I think I used to run the Chaos 12 meg SF2 as my default Windows synth, even in Windows Media Player, but it has been years.

And yes, you sound like you may enjoy RB if you gave it a whirl.
It is different than BiaB, but can do much of the same stuff, just in a different way (lots of different ways) .. and does some stuff better IMO

I do not work here, but the benefits are still awesome
Make your sound your own!