Issue 1: SPDIF RCA cable... Tried one of the cables I had last night running to an old Yamaha receiver (working on parent's system). It takes care of Dolby 5.1 with no trouble, but no DTS. Is this a situation of it's either on... or off? Can RCA cables handle DD5.1 and not DTS? Cable issue / Receiver decoding issue?
Maybe worth adding that this is most likely an issue with your A/V receiver not supporting DTS. If the receiver is older, then it won't support things like DTS.
I'm assuming the receiver is pre-HDMI, which is an indicator of age these days, as any receiver with HDMI connections will likely have DTS support.
Make sure you are using a decent RCA terminated cable. I have had some issues when I've used the cheapest RCA cables out there - poorly built ones. But, a 'premium' RCA cable from Monoprice always does a good job for me. The video cable (yellow RCA connector) that runs with a red and white RCA cable is also typically good enough for this duty. But, once again, the cheapest ones can have issues.
Issue 2: Sub woofer has speaker wire, no RCA output. Receiver only takes RCA. I've read it's one or the other. Can't run speaker wire into a line and vice-versa? Wondering if anyone could confirm that for me? No adapters can pull it off?
Worth correction: Subwoofers have inputs, not outputs generally speaking. Your A/V receiver has a subwoofer output which feeds the subwoofer signal, which the subwoofer then amplifies. Since the subwoofer doesn't have an internal amplifier, you would need to provide an external amplifier... OR... if the receiver supports it, run the left/right audio through the subwoofer connections, then onto the left/right main speakers.
A cheap subwoofer amplifier can be found here:
Subwoofer Plate Amplifiers in the Speaker Components Department at Parts Express | 332
You can put it into a box, or into your wall if you want, but those plate amplifiers are typically installed into the subwoofer box itself.