I would try to add a touch of distortion, oscillator Slop and a bit of noise. I do that with my Prophet 6 and it helps get into those vintage tones.
No there is no way to separate the Sub Oscillator From Oscilator one.
Remeber the OB6 is not based on a classic SEM 2 voice, 6 voice or 8 voice synth. Those synths had no presets, and each individual SEM could be tuned and manipulated individually. The OB6 is more based on something like the OBX.