there are .dls files floating around in the net, that contain some edited sounds, or comlete soundsets altogether.
I have downloaded some of them , one was 24 MB I think, and they surely sound better, but they seem to me as not that "polished" as the MS waveset which, of course, is based on the Sound Canvas.
Not sure who made it, Roland or MS but there is a little txt file somwhere in the win directories saying "Copyright Roland blah blah."
Do a search in goohle on free DLS files and you surely will stubmle to a couple.
Then you have to save the original MS dls file (GM.DLS, located in Windows\system32\Drivers) with another name (turning off the "system file overwrite protection" if you have XP), and name your file as GM.dls and put it in place in the same dir.
After that, you should be able to chose "MS sw synthsizer" as your MIDI synth and the new wavetable will sound.
In my opinion, Software synth by Yamaha was far better in sonic quality than the VSC, (it also includes effects) but it is now lost in oblivion, Rikki is right, they don't offer them any more.
On the other hand, search the net, or Kazaa, for yamaha soft synth, someone may have it.