I received the bag o' diodes on Wednesday (I don't expect I'll ever need to buy any more!). Thursday, while my lovely wife was out, I pulled the old diodes and tested them: one good, one shorted. Replaced both diodes, because why not? Fired up the amp in standby: no blown fuse!
Replaced the tubes, and turned it on again: so far, so good.
A quick note here: it's often a bad idea to power up a tube amp without the tubes in place. The reason is that the tubes bring down the voltage by drawing current. Without them in place, the voltage can be too high for the filter capacitors. I admit that I've done it before, and got away with it, but I don't recomment it. In the case of the 200S, and all Sunn amps of that vintage, voltage is not applied to the caps when the amp is in standby.
Hit the standby switch, and was treated to a dazzling light show from one of the power tubes! Killed the power, but blew another fuse. Replaced the fuse and switched the tubes around, figuring that, if it's a problem with the amp, I'd get the same result with a different tube. If the problem was the tube, then the problem should move with it. Instead, everything appeared to be fine. At this point, I'm thinking that the tube was bad, and burned out when I powered it up.
Gig tomorrow. After, I'll replace the power tubes and test the amp on a bass speaker.