Tweaks like these can provide a nice sonic enhancement to the stock product (disc player, DAC, etc.). But it only takes you so far in my listening experience.
Playback Designs, Merging and exaSound have nothing to fear from the tweaks.
The tweaks I do are not what you have heard. I don't just swap op amps and caps and resistors. Some of what I do is not done by any commercial manufacturer. The output stages and power supplies in the Exasound and Merging are no where near as good as I do. I use WA Quantum chips on caps and semiconductors, I damp heatsinks and other parts, I remove the steel bolts and plates from power transformers and lift the transformers off the chassis, I twist wires, I use audiophile fuses and in some case remove them, I have several types of exotic power supply filters, shunt regulators, pure transformer output stages, hand made hook up wire using solid core PCOCC cryoed copper in cotton and twisted and used in the direction it sounds best, hand modified Wima bypass caps using 6N wiring and marked for outside foil polarity, better clocks than Crystek, etc. etc. Every single thing I do improves the sound. These are not theoretical mods. I listen to every thing I do.
I have no doubt that serious mods to the Oppo Sonica and new Gustard X20U Pro will be way better sonically than a stock Merging or Exasound. Now you could do these same mods to those products but then you would be paying thousands of dollars more for the basic DAC and have reduced resale value. By modding inexpensive but seriously capable DACs (once modded) then you have super serious sound for little investment.
More than likely I will have a fully modded Oppo and maybe a Gustard to send to reviewers, etc. to hear the incredible sound themselves in their own home/system. Hearing is believing.