Great Amplifiers built like Tanks and pretty much straightforward to work on, Its worth experimenting with coupling caps and electrolytics however no need to go all exotic (expensive)
They sounded at their best for me with Tung-sol or RCA black plate 12BH7’s and Philips JAN 6550A’s
They sounded at their best for me with Tung-sol or RCA black plate 12BH7’s and Philips JAN 6550A’s