Barfly
I'm not using the assembler, but the debugger is more modern than MonAm.
Download from https://aminet.net/package/dev/asm/BarflyDisk2_00
How-To
Get sources and symbols in DevPac GenAm builds, even after linking
This wasn't working for me in GenAm but works great in BDebug.
Run GenAm with LINE DEBUG:
genam LINE DEBUG ALINK source.asm
Link with whatever linker you want. I used PhxAss.
Symbols will be visible in the disassembled output, and you can open the source window after you've activated the registers view:
