The main downgrade to consider is CABLES. Yes they do matter especially on analogue domain and they can change imaging and bass extension, but only for a few percent of overall performance. There are plenty of high quality and extra low-priced cables that you can acquire instead of current expencive.
I prefere to buy unterminated cables(speaker- and interconnect- as well) and terminate them myself. So some little DIY will help a lot and not only with cables. You can also brew yourself a nice passive preamp if you have large enough output from your source component(s)
You might also consider changing NOS tubes to a regular ones that are designed by manufacturer.
IMO I would downgrade my source last since I've built system basically from the source(believe it or not from the turntable cartridge) and first "victim" will be power amp.