Zdeng -- using coax digital (metal) cable between your tranport and DAC would (at most) tie the two units' grounds together (and possibly not even that, depending how Theta designed the circuitry.)
So if you don't have hum problems, I wouldn't worry about it. As for sonics, I've used ST for 15 years because, although my Wadia WT2000 CDT had all kinds of output choices, my Wadia 64.4 DAC had only a single ST input! I did however begin upgrading my ST cables a couple of years ago, with just minor improvement, UNTIL I finally sprung for an Aural Symphonics Optimism 2 ST cable ($950 msrp -- I was lucky enough to get one used ;--) What a huge difference! Really, it was like a $20k (at today's prices) component upgrade!
I've since upgraded my DAC to a Wadia 27, so I have all possible types of input/output choices, but I know I'll never part w/ my Optimism 2. It's just too good -- in fact Aural Symphonics now makes a 3 cable ST set for the Meitner CDT/DAC ($3000!) which a lot of people are raving about. I guess what I'm saying is keep your Theta equipment, it's great stuff lacking only SACD (not important to me!) and see if you can score an Optimism 2 someday, you'll be very impressed.
As for PCs, any good PC (meaning with conductors 14 AWG or larger) will do for the transport IMO. I use a 10 AWG Purist Venustas on my DAC which made an audible improvement.