Thank you both for the suggestions...
I did get it to flash red and beep, but probably not the way it's meant to in this procedure...
At first I tried with the PS/2 keyboard... No change; still the periodically flashing USB stick.
Then removed both the audio card and the video card. Plugged in an old simple card. Tried again, still the same...
After that removed CPU1 (rear one) and removed all but 1 of the memory modules. So now it has only CPU0 (front of the machine) and 1 8GB module in the lowest slot near CPU0 (as indicated in the memory load order image on the side panel.
Now I got it to flash and beep.... but only 5 times, indicating a 'Pre-video memory error' according to the service manual :(
After two sets of 5 flashes-and-beeps, the beeping stops but the flashing continues.
Also tried with adding a second DIMM in the top slot at CPU0; still the same. Reseated the first module, replaced the first module.. (Also with a different type; half of the memory is Nanya, other half is Micron (both HP branded))
Unfortunately I don't have a non-HT CPU. Got a Dell server laying around here that also had S2011 CPU's, but those are also E5-26xx 8-core ones... Maybe only a little slower then the 2690's from this machine...
