The answer is that the asynchronous sample-rate conversion does make the DAC somewhat immune to incoming jitter, but not completely. You can test this for yourself by using a really cheap S/PDIF cable to it and then a really good one. If your system is resolving you should hear a difference, I do. Whe I drive the DAC-1 from a transport and then from an Off-Ramp USB-to-S/PDIF converter, the difference is obvious to me.
However, in your case, your money is probably better spent on upgrading the upsampling clock in the DAC-1 to something with lower jitter. Even with ultra-low jitter on the incoming S/PDIF signal, the upsampling clock will add its own jitter, and this is the final step in the chain. Improving the input jitter without improving this clock first is not recommended.
Steve N.
Empirical Audio
However, in your case, your money is probably better spent on upgrading the upsampling clock in the DAC-1 to something with lower jitter. Even with ultra-low jitter on the incoming S/PDIF signal, the upsampling clock will add its own jitter, and this is the final step in the chain. Improving the input jitter without improving this clock first is not recommended.
Steve N.
Empirical Audio