@gerryjazzman Thanks again for posting your comprehensive info & diagrams, I really appreciate it.
I opened up my WT300 and bridged the connection between R19 and the LED1 anode with an alligator clip. For the record, I can confirm that this does indeed defeat the compressor!
The amp sounds quite a bit better without compression being tied to the gain level. After putting in some playing time with the compressor off, I'm actually a bit baffled as to why the WT300 was designed to have the compressor and gain twinned in the way it is. Perhaps good compressor pedals weren't available in the 90s? Anyway, I'd still like to leave the stock compression available via a switch, but I've decided to mount it on the back of the amp. I expect I'll run the amp with the compressor off likely 100% of the time, it's not critical for my usage that the toggle switch be available on the front panel.
Some parts were delayed in the mail, but I'll post an update here once the mod is done & dusted!