I have the Emulator X Studio which includes Proteus. It sounds good except for the one thing you're talking about here and that is it's not GM. It has a GM soundbank but it's the old (older than the VSC I think)Emu soundchip that they put into the equally old 80's Soundblaster soundcards. It's about as good as the chip on your motherboard, I doubt you're going to like it much. To use the much better non GM Proteus sounds you have to pick what patch you want to use for what instrument in Biab manually. It won't just happen by hitting play like it will with your Ketron or the VSC.

As to your question asking why won't someone build a good quality GM synth the answer is nobody cares about GM. It's only there as a convenience for non musicians to play games or listen to occasional midi files embedded in a website. Or, Biab people like us. Pros or semi pros who use audio/midi recording software want the very best sounds they can get and are willing just like Notes does, to pick and choose the best sounds from many different synths. You don't need GM for that. There's only a very few good sounding GM soundbanks out there and you already have one of the best in that Ketron. I've also got a ton of hardware and 4 top line softsynth packages but for casual GM use with Biab I like the Forte DXi for $40. To me it's a tad better than the TTS-1 but YMMV of course.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.