I have both an OB-6 and Prophet Rev2, and I noticed something interesting today: Starting from an init patch on both, mix both OSCs, put a reverse saw LFO on the cutoff and adjust to create a sweeping effect, and generally try to get a fat "dohwww" bass sound that sounds similar on both. The OB-6 just comes out of the gate with a lot richer harmonics and noticably more character in the filter. The Rev2 takes a lot of tweaking but can get to almost the same amount of warmth, and with about 5 times the effort can sound surprisingly similar to the OB-6.
However, as I play that sound lower and lower, the harmonics start to fade from the OB-6 bass -- the low frequencies just become inaudible and you hear more crackling of the filter than anything else. The Rev2 can go about a full octave lower before the bass patch becomes unusable, it holds its warmth better across a longer range.
My question is --- is this characteristic of the difference in Curtis vs. SEM filters? Or is it more of a DCO / VCO thing?