Hiya:
First off, Andrew, congratulations on the SP-600! That is my main HF receiver, it is much easier to "cruise" the bands with than the R-390A, which I prefer for sitting on a specific frequency. At my main listening post, those two are in service almost continuously, at least while I am present at the dials!
Meanwhile, I also have a SRR-21 & a SRR-23. The SRR-23 has never worked, but seems to be complete & intact, but the SRR-21 worked well for years, that is, until something (transformer) smoked while the radio was plugged in but turned OFF! Grrrrrrr! They are VERY difficult to work on, especially without the extender boards etc. and yes, they contain subminiature tubes soldered into plug-in boards. Another cool feature that they have is a "projected dial display" on the ground glass directly above the visible dial. They are fairly rare anymore, but not impossible to find. Fair Radio Sales sells a copy of the manual for them for somewhere around $60, I have a copy myself including the schematics etc. Basically, they were replaced by the R-390 / R-390A radios. Eventually, perhaps I'll get around to trying to service mine, but....In any event, the SP-600 & the R-390A are entirely better receivers anyhow, IMHO, and the R-390A has marvelous audio via the (adjustable & metered) line level output into a stereo amp & speakers. Best wishes etc. & good DX!