"If I insert (between my source and the pre-pro), a new DAC that’s SUPERIOR to the pre-pro’s DAC, I assume that the pre-pro would limit or degrade the SQ of the new DAC. Correct? Is feeding the signal from the pre-pro an option in this case?"
ANY preamp will add some distortion, compression and noise to the analog signal, but you need some way of controlling volume and achieving a low-output impedance drive to your amps in order to get decent dynamics.
Most decent DACs (over $1K) will outperform the DAC in a SS processor. All you need to do is feed the analog outs from the DAC to the SS proc and control volume with the SS proc. This will probably improve the SQ over the SS Proc. internal DAC, but you have the preamp adding noise and distortion.
A better solution is to get a DAC with good output drive and replace the SS proc. with a transformer-based linestage like a Music First.
Another option is to get a DAC with good output drive and a good volume control technology and replace the SS proc. This is what I do. In order to insert other analog sources such as home theater or vinyl, I use a Final Drive after the DAC and before the amps.