As an alternative, partsexpress has optical/coaxial converters for under $20. I don't have any experience with such devices, but I believe they should work. I realize this won't give you DTS like you said you wanted, but I thought I'd just throw this out there anyways.
I get that with one of my amps as well. I've been told it's probably just the output capacitors from the amp releasing their stored charge, and that it really isn't harmful.
I was in a similar situation, I had a Marantz 5200 (using direct mode), a pretty decent pair of speakers, and was using the DACs in the 5200 with my DVD player. I did a lot of work positioning the speakers, getting better interconnects/wires etc., yet I still wasn't getting the kind of music I...