In the picture you can see that the leads of the mains cable go directly to the component board (brown and blue lead, respectively), The ground lead (yellow/green) goes to a soldering tag attached to one of the screws holding the top transformers in place.
The two small transformers on top are supplying about 200V AC to each channels voltage multiplier. The connections here are very simple, as you can see in the schematics.
(Never mind the red marks, it is where I cut and re-connected the secondary leads when making an outboard AC supply instead. This I did to get rid of hum and "magnetic interference" -and also to be able make the bias voltage adjustable, as mentioned earlier ).
Back to the mains leads. Electrically one of the leads is connected through the fuse to the primary winding of the two small transformers, the other goes to the other end of the same winding. (The route through the component board is electrically un-essential, except for the connection to the (un-essential) lamp (L).
Good luck! Olaf
May I add that in my opinion the ESP-9 is extremely well suited to the use of EQ, due to the surprising big headroom in the bass. (actually I mostly use a 3-band conventional but high quality tone-control unit). Otherwise the dominating mid tends to give the sound a rather tiresome, hard and somewhat metallic character. Corrected –and with the cans supplied with fresh foam- these become fabulous sounding headphones, if you ask me: Very objective, powerful and engaging (unless if airiness and big spatial presentation is primarily what engages you, that is). I am tempted to use the word "masculine" about the sound. They are my favorite headphones nowadays!
They ideally need a decent amp to feed the step up transformers, but interestingly the power section of an old and inexpensive NAD 3020 suites them quite well, adding a tad air, mild upper bass boost and a little (feminine) softness.
(In any case their high weight and low comfort will give ones masculinity a certain boost..).