I've heard enough modded knockoff C800-Gs to conlcude, they're not worth it. The rare and extremely expensive tubes are a big factor in the sound/performance and each tube will you set you back far enough that you're better off selling it and buying a better mic instead.
I'm telling you this as someone who hoards original Sony C800-G tubes whenever I find them for our C800-G at the studio and I consider selling it sometimes based solely on the cost and hassle of finding them.
They were made by 1 factory in Ukraine owned by 2 brothers (according to my rep at Sony). 1 of the brothers died and the remaining brother closed up the business. The final run of C800-Gs were supposedly made possible because someone at Sony found a lost stock of tubes in one of their warehouses. They made a last limited run, saved enough for warranty repairs, then listed the remaining tubes for sale on their website. It was something ridiculously low like 13 tubes, if I remember correctly.
I got a phone call telling me they'd be posted on the Sony website in 30 minutes. I wasn't the only person who got a phone call because I refreshed the page over and over on my phone for half an hour and in that half a second free-for-all I was only able to get 1 tube for, I think, $500. Then, it was back to checking eBay over and over to pay as much as $1,200 per tube. Usually $1k each. The PSU uses reject tubes that weren't good enough to earn the part number for the mic itself but are good enough for the PSU. I've tried all the knockoffs and some mods of them, hoping to get lucky enough to find an alternative with readily available tubes that I could sell the Sony, buy a pair of, and have plenty funds left for other things around the studio, but none of them are anything like the real C800-G. A Manley reference will get you much closer than any of them.
The Sony C800-G is always smooth, never harsh, even when the source itself is a little harsh. Every vocal immediately sounds polished and compressed, making EQ/compression optional. It handles drastic sudden SPL changes, like going from a soft whisper immediately into belting out the next syllable like Mariah Carey, as smooth and without distortion as if it never happened (other than the volume difference, of course). Despite its total lack of harshness, its still somehow the brightest clearest most detailed mic there is. It destroys the Neumann U47 in that regard and falls short of it nowhere (U47 is still great for its distinct character though). I'd rank the Neumann M49 just below it followed by the Elam 251 and AKG C12. Its like if the C12 or 251 were upgraded in every way.
While I suspect someone here has the ability to replicate it right, it would cost a lot of money for them to do so, and even then that person would have to be someone who puts David Bock to shame... Possible but unlikely.
