You've tried a lot already... i have to stress that you have to try a few more ram modules imho. Sometimes these old boards can be extremely picky. Bad ram i throw away so the ones i have SHOULD be working. But every now and then a board comes along that is not willing to start up and only with one particular ram there is life.

Try EDO and Sdram if you have both. I had a s7 board once that only worked with EDO and AT psu, never got it running with Sdram and Atx psu.

Also make sure you do the cmos reset the right way: do unplug the psu and do remove the battery when resetting the jumper.

Next thing i would do is use the pci graphics card in every single pci slot, you never know. Also try an Intel cpu and not only Amd (Higher voltage). Try the lower 66fsb instead of the 100fsb (easier for the board).

If you have more psu's then try them, even with psu's they can sometimes be picky. 2 might not be enough.

I have the same AX59pro and it is not picky at all so it shouldn't be a problem but you just never know.

Caps can be the problem and indeed it cannot always be seen. But in most cases if they are that bad there should be a bulging one somewhere.
The board is worth it so i would do the recap if nothing else works.

I had the exact same problem with one Asus P3B-F and a Gigabyte GA-5AX and after trying wayyyyyy to long i tossed them out (otherwise i kept trying every now and then).
I never tried recapping them as i didn't know how to do it. Now i do but those boards are gone.

Good luck! And report back if you find the problem or get it working.

If you have a programmer, I would flash the latest BIOS.

I have one of these boards myself and it does the SAME thing. Using a 4-digit POST code testing card I get - - - - on the display, no matter what I do. No beeps, nothing. I can run the system with all the components or run it without anything else connected at all (just the board and PSU, no CPU) and it still does the same thing. I've tried multiple CPUs, multiple RAM sticks, everything. Since there are no beeps or any POST codes at all, I'm thinking it is failing very early in the process.

Does this sound like a BIOS problem?

If so, my best bet is probably to get a less desirable board that I have laying around and hot-swap the BIOS chips to program one for this board. Are all BIOS chips from this time period likely to be compatible with all sockets as long as they are physically the same size and number of pins?

EDIT: Apparently, this board just needed to hear the threat of having its BIOS hot swapped into another system. Strangely, after disconnecting the keyboard, putting in yet another stick of SDRAM and reinserting a CPU, it started giving beep codes! When I inserted some EDO RAM (it has SIMM and DIMM slots) it booted right up without a hitch! I'll have to check the BIOS settings to figure out why it doesn't seem to want to POST with SDRAM, but the BIOS seems to be fine.

I have no idea why it wouldn't come on before, but I'm happy its working. The board is absolutely pristine. It may not have ever even been used.

EDIT2: Woohoo! Apparently the crazy DRAM speed select jumpers (which are split between two different banks of jumpers) were set to run at the AGP Clock, rather than the CPU clock. Some of my SDRAM still didn't want to run in this board but the Samsung 128MB PC-100 CAS3 stick I've been using for testing booted up perfectly after switching the DRAM jumper to CPU Clock (100Mhz in this case). Still no idea why it was totally dead earlier, but I'll keep playing with it just in case the problem returns. This seems like a nice board though. FSB overclocking settings on the board, well documented jumper settings, nice BIOS options... it even has a "Y2K CMOS Update" enabled/disabled setting in the Chipset Features Setup. 🤣