Man that is sweet, congrats! What speakers are you using with it? How's it compare to your beloved 6060?
I have Klipsch Forte with Crites upgrades. I will hook up to 400 after Xmas (I work 7 days a week right now).
The Sony 6060 is insanely good sonically for SS so the 400 will have a serious run for top place. But, I am prepared. The 400 has factory stock Telefunken & Mullard tubes with little hours use. I have eight NOS Brimar 12AX7 known for top sonic performance. I also have a set of NOS Sylvania 7868. Tube swapping will tune this 400 sonically.
If the Sony wins at this point, I may upgrade this 400. It is kind of a tough decision as this 400 is a museum piece that will play music as Avery wanted it.
The 400 power supply has about 10 volts AC ripple as designed. A PS doubler with two new 680uF into a LC for high B+ & another LC for low B+ should provide a quieter 'black' background & more airy soundstage due to less IMD products present. (A thermistor on the 120 AC line will control the higher in-rush current). However, sometimes some low level AC can sound better.
Another area for mods would be the coupling caps. Both the finals & especially the tone stack could be upgraded. The tone stack uses a ceramic package that cries for an upgrade. The Ero 7868 power tube coupling caps are a good product. I believe only the 400 came with these superior coupling capacitors.
I will report back the performance & detail upgrades if performed. I never seen anyone address the PS with some filter chokes. It may be a relevation or may not.