It may function and play music but unless it's been completely rebuilt then it doesn't work perfectly. All the ES-1 and ES-2 amps have the same problems, single filament supply which is grounded and the tubes really don't like that with -450V on the cathode, the G3 isn't connected on the output tubes which causes some interesting issues, the bias supplies aren't even close to what they should be and the offset adjustment doesn't work so the amp is always stuck at -110V. Then we can add to that the fun option such as a digital voltmeter to adjust the amp (doesn't work and causes a bunch of issues), the digital volume control which is a huge kit from China which often didn't fit the boxes very well and called for some interesting wiring.
The whole myth that the SP amps were once good and then deteriorated simply isn't true. The first ES-1 amp was just as fubar as the last ones. The ones with the most custom stuff are the worst though and any that were sent in for repair Mikhail often stole parts from and substituted with some very cheap crap.
That's just too sad.