Cost effective mods for a Sony SCD-777ES

Ditto Czbbcl's recommendations for modders. If you can do the work yourself and want to work incrementally, start with the Superclock 4, replace the AD712 op amps in the output stage, replace select caps/resistors in the output stage signal path, remove the AC filters, and star-ground each board. There are posts on AA SACD Forum regarding these changes.

I agree with Tvad and Drubin. Use the SCD 777 as transport (excellent one).
I use mine for transport and use an out board DAC with excellent result. With the cost of mods, you can get a good DAC such as Musical Fidelity Trivista, or a tube DAC easily.
This way also give you the flexibility down the road if you like to change the dac for different sound.

The Sony DAC is not a gating factor. With clock replacement, power mods, & a swap of output stages to VSEI or buffered RAM coupling transformers, the 777ES will approach SOTA.