Older versions of Doom can't go higher than IRQ 9. Try switching IRQs.

Thank you for your suggestion, Rawit . I just tried with IRQ 5 instead of 11 without success .
I also tried with DMA channel 3 instead of 6 and 7 .
I then disabled L1 and L2 caches, but did not get lucky .

My gut feeling says the issue has something to do with DMA and dynamic patch caching not working well over a bridged ISA bus .
I already removed the CT3670 to run the low IRQ tests and will soon be removing the Audiotrix 3DXG and trying the GUS PnP in different ISA slots (doubt that will help). Next I will try to find where I stashed my Gus Max and try with that .

Is anybody successfully running a GUS PnP or classic in a bridged ISA slot using an ITE 8888 variant ?

Update :
Just found this thread --> Re: Short on ISA slots? Try this.
Apparently GF1 and ITE8888 are not a match made in heaven .

I tried playmidi.exe for GF1 based GUSes and get a 'No response from DMA channel' when it starts trying to load patches into GUS RAM .

Either ISA DMA just does not work for the GUS PnP on my bridged setup or I have a defective GUS PnP (I know for a fact it worked fine about 15 years ago) . Luckily, I have a spare one I can try when I feel motivated enough (also known working 15 years ago .

If that does not work out, I will probably try reading the ITE8888's datasheet, maybe there is some register I can set .
I already tried the registers shown in this video of somebody using an offboard ITE8888 bridge https://www.youtube.com/watch?v=nlYXbxQneG8 , but had no success . Anyway AWE32 already works for me through the bridge . Maybe I will find something useful in the datasheet (if I can understand any of it).

P.S. If an admin is reading this, the thread (almost a monologue,actually) would probably be a better fit (and possibly interest more people) in the Sound sub-forum .

While looking for my spare GUS PnP, I managed to find my old GUS Classic 3.73 and... it works in Doom, Doom 2 and Star Control 2 and everything else I have tried so far !

So it looks to be an incompatibility between my board's (Ipox 3ETI23) ISA bridge implementation and the Interwave-based GUS PnP .

Update: Vertigo Nature works too (needed to set a lower IRQ) and playmidi works fine too . The only things not working are midi synth under Windows 98SE (no sound, but GF1 wave-out works) and Gravis' setup utility DMA tests . I can definitely live with that .

Now If only I could find the GUS Max to try it out .

Glad you solved it! Well kinda solved it... Still curious what causes this though, as an Interwave contains a GF1. When initialized in Legacy mode things shouldn't be that different AFAIK. Is it perhaps a ISA speed issue?

Under Windows 98SE in most applications the patches aren't loaded, so it plays the MIDI without instruments. Windows 95 doesn't have this problem. I ran into this back in the day, but didn't find a fix.

Is it perhaps a ISA speed issue?

Maybe. AFAIK, ITE8888 clocks the ISA bus at PCICLK/4 , so 8.25 MHz . A late vintage ISA card should not have issues with that, one would think . I still think something is not quite right with ISA DMA on this motherboard, given that even the GUS classic's Setup fails DMA tests (admittedly, that could be a timing issue to the relatively high CPU speed of 1.4GHz)

Under Windows 98SE in most applications the patches aren't loaded, so it plays the MIDI without instruments. Windows 95 doesn't have this problem. I ran into this back in the day, but didn't find a fix.

Good to know it is not an ISA bridge issue . I never actually ran a GUS classic under 98SE back in the day . I bought a GUS PnP as soon as they came out (1996, I think). I simply decoded to forego installing GUS drivers under Windows as there is not much of a point with 2 other working sound cards and so little GUS specific stuff for Windows.