I thought of these upgrades also, but at the end I took my DH 200 and simply replaced the big caps with ones a bit larger (Qua Co offers these, but Mouser and others have them) and also replaced all the electrolytics on each board.
I also bought like 10 or 20 of each semiconductor that controls the DC offset and matched/replaced them to get the DC below 15 uV.
These upgrades to the standard boards made a nice improvement in sound, at least compared to 30 year old components.
I also learned a lot by doing the project. I'm pretty satisfied with the sound, maybe someday I'll take the board upgrade path, but for now am quite happy. Worth thinking about in my opinion..