After the value-relations copied from the Interwave PDF into the pan table(~*2048), the music sounds right but to actually verify this for sure it would be preferable to create some sort of test MOD file to play that varies the pan values or use the gus sdk to test the pan modes and values, then record from both real gus and the emulator.
However as far as my ears go, the changes in the last post fix the pan issue for the games/demos mentioned.
The glitch bug and the incorrect playback speeds are problems I can't really fix as they require more indepth knowledge than I have time/interest to get involved with.
Someone who has programmed directly to the GUS sdk would probably be the best person to fix those as they could create more easily repeatable tests.
the GUS vs ACE is more about the belief/observations that the ACE sounds is more "clean". In the recorded videos this appears to be case. So to "emulate" the GUS instead of ACE, you'd maybe need some more EQ or high pass filtering. This is kinda similar deal as with Amiga 500 having filtering modes for the audio. WinUAE has a toggle for the filter and the effect is more pronounced.
And as far as the gus.cpp goes, I don't see any sign of emulating the effects mentioned in the InterWave IC Am78C201 Programmers Guide v2, (SDK) 1996.pdf chapter 7 ... There's few games that load patches into GUS and use it as synth (instead of "dumb pipe for pcm"), maybe those patches contain parameters that change the effects. I don't know how to implement that effects stuff. The Settlers video linked above is a demo of this - there's way too much reverb in it and desc says "custom tunings" - as the sounds are same as original but just more reverb, it suggests the video author found a way to increase reverb for the patches used by the game.
re: test tone stuff: I don't have GUS at hand, all I could do was to check agains the youtube videos and the dosbox gus sounds good now after changes posted earlier.
However it would still be really nice to have some way to incorporate the freeverb (comes with the munt code, already in dosboxX) into the dosbox mixer. So you could configure dosbox mixer to add reverb to each sound source. This would allow the Adlib emulation to be then matched by ear (using the overlay you mentioned) to sound like this recording from Adlib Gold with surround module (and maybe if someone has time the adlib code could be changed to emulate the surround module, but would be much less work to just add the reverb adjustment to the mixer):
https://www.youtube.com/watch?v=71icIsoA_v0&l … B135F33&index=7
Another nice to have would be to have ASIO support (so you could then get lower latency with ASIO4ALL). Currently I think dosbox uses DirectX for audio and it has notorious latency (try running AXS synth in dosbox with GUS)
PS. with the pan table fixed, check out TTD and Epic Pinball. Both sound nicer than anything I've seen uploaded to youtube atm. (There current uploads are incorrect - not enough stereo width)
edit: found another emulation attempt here but might not do any difference to DosBox - it doesn't seem to have the pantable from the pdf either. https://github.com/qemu/qemu/tree/master/hw/audio