After some more thorough testing, I determined that the volume of the 3320 VCF was a little on the quiet side, noticeably lower than my other modules. Putting this down to the use of 100k resistors rather than the 91k specified on the circuit board, I decided to replace them and am happy to report this did the trick!
One other interesting thing I noticed is that even with the resonance turned all the way down, and cut-off all the way up, the 3320 VCF does add a little bit of colour to the incoming signal, softening the edges and (dare I use a cliché) making it sound warmer.
Resonance seems to self-oscillate nicely from about 50% onwards and seems pretty easy to tune. There are also optional solder points on the PCB for a 1v/Oct input, so I’ve no doubt it could be quite a capable oscillator too.
All in all, a great addition to my modular synth and looking forward to trying out some other Guru Gara Synth modules in the future!