No idea. Depends on DAC chips, architecture, the output stage of the Sony, shielding, power supplies, whether the Rotel converts analog signals to digital, etc. The Sony's probably a decent transport so I think you're on the right track with a DAC(especially since you don't hear much of a difference between the Sony and the Rotel), but I'd go with a coax digital connection over toslink if possible unless you're using more than 1.5m length.
I also don't know about the CAL Sigma DAC, although I know CAL makes some good stuff. I do know that the EVS Millennium DAC II competes with some of the best out there, and at $1050 and a 30-day risk-free trial period my recommendation is you give it a try. Given the level of detail and soundstaging capabilities of the Thiels(I've always liked the CS1.5s too) I think you'll be amazed at what the EVS DAC will do for your system. I'm running my EVS DAC through a Rotel RSP980 and the improvement was not subtle over a modified MSB Link DAC. Check out the EVS website at www.tweakaudio.com for more info, and the reviews on audioreview.com and stereotimes.com to see more complete descriptions and comparisons. (If the RTC965 digitizes analog signals I wouldn't bother with a DAC until I got a new pre/pro or a stereo preamp with an HT passthrough). Best of luck.
Tim
I also don't know about the CAL Sigma DAC, although I know CAL makes some good stuff. I do know that the EVS Millennium DAC II competes with some of the best out there, and at $1050 and a 30-day risk-free trial period my recommendation is you give it a try. Given the level of detail and soundstaging capabilities of the Thiels(I've always liked the CS1.5s too) I think you'll be amazed at what the EVS DAC will do for your system. I'm running my EVS DAC through a Rotel RSP980 and the improvement was not subtle over a modified MSB Link DAC. Check out the EVS website at www.tweakaudio.com for more info, and the reviews on audioreview.com and stereotimes.com to see more complete descriptions and comparisons. (If the RTC965 digitizes analog signals I wouldn't bother with a DAC until I got a new pre/pro or a stereo preamp with an HT passthrough). Best of luck.
Tim