Thanks for all the advice folks, at the moment it looks as if my cyberstorm died at some point.
I've replaced the oscillator with a known working one, and examined over the entire surface with a strong magnifying glass. No damage I didn't know about before, and no little bits of wire/solder/fluff between the legs of any chips/ics.
I've tried reworking every chip, transceiver, capacitor, resistor, resistor pack and transistor on the surface (excluding the big Xilinx one, because the legs are way too close together for me to rework :-P), and the connector slot, and scsi slot solder connections, but no effect. Haven't done the simm-slot connectors yet.
I've tried it with ram, without ram, with 060, with 040 (changed resistors and voltage, added and removed LT1085 voltage regulator respectively) - no effect. Have confirmed the 040 and 060 work on different boards.
Have treble checked the jumper settings on the a4000 just to be sure (and have tried on both INT and EXT clock settings). The cpu gets warm (no more than expected) as do the mach chips, and the big square blocky ones.
The zorro scandoubler outputs a signal, but the screen just stays black - no yellow, green or red screen. Cannot access the early-boot menu. Am in the process of building a A2000-A4000 keyboard adapter, so cannot yet advise whether caps-lock light is working. Don't have an LED handy to check the behaviour of the power light either, bah!
Same behaviour on the a3000. Haven't got a cpu-board for the a4000 to double check the mobo with at the moment, but will pick one up next weekend.
Any suggestions?