DId you get channel imbalance at the start when you received them?
Definitely not the amp's fault this seem's to be the 202's. If you can let them continuously play and hopefully it cures itself (based on spritzers advice, recommendation and experience as the same he told me), then you're going to help a lot of folks including myself when we run into "temporary channel imbalance".
A 252A will power the 407, just not to it's potential.
And yeah, as I told you while ago, I'am no longer buying used vintage gear from Yahoo Japan as well (except deals too good to pass on, pads, accessories, stands and NIB's amps/headphones). The last items I purchased from them left a sour taste to my experience, one being channel imbalance, amp stopped working, squealing noise out of the second 404 I bought but managed to fix it. Those Japanese sellers are never clear with descriptions and pictures of the items. Google translate also sucks arse and no returns is a PITA.
Don't sell off the 202 yet.