I purchased a Theta Pearl transport about 4 months ago. At the time, I was considering getting the Sonic Frontiers SFT-1 and the Theta Data Basics also. I hadn't actually heard any of these units, but ended up getting the Pearl because I found a good deal here from a nice person.
Everything I read about the sonic characteristics of these units seemed to indicate that the SFT-1 had a more laid back, mellower sound than the Thetas. The Pearl is working wonderfully for me hooked into a Bel Canto DAC 1.1. Like many people I like smooth natural sound, but also enjoy the dynamics the Theta imparts. The Pearl has a huge soundstage which compliments my ProAc Tablette 50 Signatures wonderfully. And even though I'm using a solid state Simaudio Moon I-5, the sound isn't harsh or analytical. It's on the warm side and dynamic.
I don't think you can go wrong with any of the units you're considering. Used, the Sonic Frontiers may be about $400-500 more than a Pearl, but I don't think it's money wasted either. Build quality on all these transports is very high. If you're looking for something less aggressive, the SFT-1 may be the ticket. That being said, I personally don't find the Theta to be screaming for attention either. It is smooth when it should be, and dynamic when called upon.
At the time of my purchase I spoke with Ed Deitemeier with Theta Digital. He told me they were having a tough time locating parts for the Theta Data Basic Phillips transports, but that the Pioneer Stable Platter being used by the Jade, Pearl and Miles was still in production. It seems now you've found that the Pioneer mechanism is being discountinued. I would do a search in these forums to find out about any reliability issues regarding the Stable Platter if you're inclined to go that way.
You have a really nice system. I wish I could tell you something about the Proceed, but I've never heard it nor have read anything about them. I think the choice between the Theta and the Sonic Frontiers comes down to what sound qualities you're looking for in a transport and how you want it to help shape the components you already have.
Good luck and happy hunting!