I changed the two large capacitors, but unfortunately the hum is still there.
In case it makes a difference, the sound comes from, or is loudest at, the toroidal transformer. Any other suggestions to try?
Finding replacements locally was not easy, as most stores don't seem to stock 10000 uF units. Finally one store had some: Illinois Capacitor (IC) 10000 uF 50V 85C 35x35mm snap-in. They also had 35V-rated ones, but they were a fair bit smaller (maybe 30x30mm or less, I didn't measure) than the original's 35x30mm.
The Samxons both measured at ~12000 uF by my multimeter. At the store I also asked them to use an ESR meter and the resistance was very low, better than the replacements I bought. So it seems the Samxons are fine. The guy at the store was kind enough to suggest I could return the ones I bought if they didn't solve my problem.
For reference, I'll drop in this thread with lots of info and photos on the Z-2300 and its components: https://www.electro-tech-online.com/...former.114427/
As for the disassembly, I managed to remove the board by cutting only one wire--ironically the black wire on which I had previously removed a crimp and soldered together. It was quite a pain: dozens of screws, nuts, tons of glue all over securing many components, and even something like epoxy under the D-Sub connector. There is also what I assume is thermal paste applied with some strange plastic between each amp IC and the heatsink. That made a bit of a mess. I wonder if that needs to be reapplied after disassembly...
I also managed to remove the speaker wire without removing the speaker (getting the grill off is a pain since apparently it is glued). It required a lot of patience, contortions, and using a webcam for remote viewing. I've included a (blurry, not enough space to focus!) photo of the inside of the (upside down) sub, which at least shows the wire order: red then black, going clockwise from behind the speaker.
I'm really not looking forward to disassembling everything again to swap out the capacitors again... and mostly just disappointed the problem isn't solved.