HP and Compac's are a PINA to update.
I picked up 2 old XW9400 dual processor motherboards and had a terrible time getting them up and running. Got 2 matched sets of 6 core CPU's for them and couldn't get them going.
Finally, to get them going, I had to purchase a matched set of dual-core CPU's and got both motherboards to work.
I had to update the BIOS's on both to get the motherboard to except the 6 core CPU's.
Put the original CPU back in, then reset it again as you did in post 11 above. Check that the pins in the socket are not misaligned or that the pins are free of CPU grease. Before you set in the old CPU.
The HP xw9400 motherboard's that I have are finicky. Make sure all the memory chips are seated correctly, then reset again as in post 11.
I also have a Tyan S2927, also a dual socket motherboard, and the guy I bought it from shipped it without the CPU covers. It came with 2 CPU heat sinks, but he didn't clean off the old CPU grease on them, and the semi-dried grease got into the socket pins. Although he rapped the heat sinks in bubble wrap, one of the heat sink brackets slipped out and bent some of the socket pins.
I had to use a whole bottle of isopropyl alcohol in a sprayer to clean out the semi-dried CPU grease in the sockets.
I used an X-Acto knife with a magnifying glass to re-bend or re-seat the socket pins. Works perfectly now.
So take your time and use a good light and a magnifying glass to check those pins in the CPU socket. Re-seat the old CPU then try again.