At least there is a tool called SBTimbre that lets you edit OPL instruments and it supports MIDI input to test the sounds.

Thanks Jepael - that is an awesome app that looks like it will work perfectly. Looking into SBTimbre, I found this page http://www.oplx.com/opl2.htm which is full of trackers/editors

A lot of them are not really what I'm looking for but the nostalgia factor was too much to resist spending time in each one of these interfaces. Had to download em all 😀

It would be awesome if something like SBTimbre existed, but for OPL3 cards. I've played around a bit with Adlib Tracker ][ but the UI is super confusing for me. 😳

And they "dare say it's the most user friendly interface" 🤣

That's another cool option though! Luckily they opensourced it at some point- maybe there's a way to get some of the core functionality without the tracker part, and add MIDI support for keyboards.

https://github.com/ijsf/at2 specifcially https://github.com/ijsf/at2/blob/master/adt2opl3.pas

It looks like it was compiled with TMT FrameworkPascal for DOS for the pascal code, TASM for the assembly code, and GCC for the C code.

Not sure if it or a subsection of it would compile with Turbo Pascal or Lazarus, but it'd certainly make for an interesting project.

The original Adlib Visual Composer (1987) came with an application for creating custom patches (Instrument Maker) that could be played with a MIDI keyboard. Could give that a try.

Adlib Tracker II also has a "Midi Synth Mode" which sounds like it does what you want, but it's not in the current version so you have to download an older copy. Scroll down to get 2.3.52.

Awesome suggestions xjas - gave both a download and will try - thank you!

Also, I found this video of a guy doing something pretty much what I want - although it would be even cooler if it was in DOS: https://youtu.be/tczvrrAQ2kw?t=98

Had to replay like 5 times and pause at just the right moment to see what DAW he's using, it appears he's using Rosegarden: http://www.rosegardenmusic.com/

He's using the Crystal CS4236, which is actually how I found the video - based on another thread I saw ( OPL3 implementations (ESFM, Crystal FM, dual OPL3 etc) ) people discussing OPL3 implementations, (never knew there were so many) I ended up picking one of these up, and am looking forward to using it.

Found some videos - I really like the bass on this card - looking forward to trying it: https://youtu.be/_ibdIm-ZBB4?t=174

just wanted to add in case anyone's interested - this looks like a windows based app to do the same thing.
again, would be cooler if it was in DOS, but beggars can't be choosers:
http://fm801.kewl.org/app/about/