The "U" in USB stands for Universal. IEEE 1394 (Firewire) is NOT universal. Most motherboards have dropped support for it. And so are most DAC vendors, sadly, have too. Universality often wins out over quality.
Motherboard based Toslink & RCA SPDIF signal paths are cheaply made, and sound that way... to me, at least. Even with the addition of a galvanic isolation transformer. I have listened to a dozen or more of them feeding a WEISS DAC 202, among others. (This was my job for a period of time) Consider that the entire motherboard costs $100, and they have to include a lot of other parts for those 100 dollars.
I'm not sure how to best invest $200. What inputs do you have on your "old D/A converter"? For less than your budget, a USB based M2Tech HiFace2 will get you RCA or BNC SPDIF out with quality clocks and galvanic isolation.
Motherboard based Toslink & RCA SPDIF signal paths are cheaply made, and sound that way... to me, at least. Even with the addition of a galvanic isolation transformer. I have listened to a dozen or more of them feeding a WEISS DAC 202, among others. (This was my job for a period of time) Consider that the entire motherboard costs $100, and they have to include a lot of other parts for those 100 dollars.
I'm not sure how to best invest $200. What inputs do you have on your "old D/A converter"? For less than your budget, a USB based M2Tech HiFace2 will get you RCA or BNC SPDIF out with quality clocks and galvanic isolation.