I just uploaded an example of me noodling with my OBX Portamento to give you an idea of what I am talking about:
https://soundcloud.com/johhnyg/obx-portamento-example
Sounds lovely for sure.
Just personally I am not sure I hear that difference where voices chase different speed of pitch.
There are some slower dives, that more feel like pitch wheel but not sure which cababilities OBX has controlling portamento and voice stealing.
But from description I think I know what you mean.
I have an old favorite using pitch envelope down just a bit in many presets, like when using slide on guitar you hit fret below and then up to pitch. If doing only on one oscillator you get some nice phasing as well.
Doing this with Unison setting and glide on top as well and setting accurary Other:wild so parameters for each voice are varied quite a bit. Unison Legato off. Some Key/seq source to glide time should get little differences on various places onkeyboard, or if one wants using voices source instead.
If unison do full duplicate voices, with own envelope trigger each voice and varied parameters each voice one would think that would help. It certainly did as I changed to Wild on parameters.
Overall I would like way more accuracy settings and calibration as well where you want to the base to be, how accurate.
On digital pianos I like to use some stretch tunings to deviate from equal temperament to avoid harmonic beating a bit in most used octaves. Seems they already built some deviations into PB doing the calibration. Three voices are tuned over equal as I checked at some time, and other three below - that cannot be a coincidence, I think.
But good FR that made me fiddle more with Unison today. Endless fun....