@kenc51: Thanks for the response and the welcome!
BIOS is current.
Yes, I've tried all four sticks separately, one stick at a time, in the DIMM 0, Channel A slot - all give the same results - three beeps. Conversely, I've tried all four separately in the DIMM 0, Channel B slot, normal boot but slow (working single channel).
Further testing shows that I get three beeps if I try to populate either the DIMM 0 or DIMM 1 slot in Channel A! But both slots in Channel B work fine, just slow. In other words, Channel A simply doesn't seem to be working at all.
So, all sticks seem OK, but entire Channel A has failed.
I appreciate the manual bios suggestions, but I think this has got to be a mobo fault (you agree?). Will try manual bios configuration, though, just to see.
Any other tests/procedures that might be useful/conclusive?