Hi algrove,
Here's a slightly different perspective:
1. If you've already rendered your Pentax captures to Tiffs, why would you bother trying to process them further with (anyone's brand of) software designed/intended for raw file processing. Why not use "the real thing"?
Finalize them with Photoshop, as that's what it was originally designed for? Now after 20+ years of development, it has tools & capabilities obviously way beyond any make of raw processing software (of course, talking about use on rendered files here).
or
2. If you want to go back & re-process your Pentax .dng capture files (yeah, maybe a herculean task, I know - but if you really want to bring out that amazing image quality...), there is a work-around to let you do this from within CaptureOne. Check out this guy's site:
CaptureFix - he's developed a hack to allow processing & even tethered capture using C1.
Disclaimer: I am not associated with this in any way, and not even clear on whether there might be any possible hazards, legal or otherwise, going this route.
I did briefly try an earlier version of his hack - it was really nice to use C1 on my Pentax .dngs, but I had issues with the color, since I didn't have a satisfactory ICC profile for Pentax (if you can master dcamprof, it might be a different story now). As you can tell, going this route would involve a fair bit of DIY technical expertise, but some folks are so inclined...
Good luck!
John