HX1200 vs retro motherboard - which component needs help?

Hi guys - this is going to be a bit of an oddball question but I'm hoping that between here and the motherboard support I can figure out what to do.

Short story is that I'm putting together a retro PC from ~ 2001 to play some older retro PC games that either don't work at all on 64-bit Windows anything, or don't work properly.

That all being said, I learned that around Pentium 4 times, current draw shifted from being mostly on the +3.3v and +5v rails to instead being mostly on the +12v rail so PSU's shifted to follow suit.

Now, I picked up a Corsair HX1200 because it has 30A on both the 3.3 and 5v rails which *should* be enough to run a Pentium III rig, and it works great on an Asus single CPU setup I was testing, but I decided to switch to a dual CPU workstation board from Tyan and I'm running into an odd issue.

With 2 CPUs installed, 1 SDRAM stick, and an AGP graphics card (nothing else at all, so only the ATX 20 pin connected), when I try to boot up all attached fans come on and stay on indefinitely, but the system doesn't POST. No beep errors or anything, just nothing at all. If I remove all RAM, it does the 'no memory' error beep. If I connect a graphics card which needs external power plugs without connecting them it does the 'VGA needs more power' beep.

So... at first I thought the board or CPUs might be bad since i know the HX1200 works just fine on that other old Asus board... but, I dug up the only ancient PSU I still have - some no-name 400w unit that claims 40A on the +5v rail, 26A on +3.3v and 12A on +12V. That thing allowed the rig to POST just fine. I populated all 4 RAM slots and 5 PCI slots as well... no problems. Eventually after getting a test setup of Windows 2000 the system becomes unstable and hangs after a few minutes but if I keep it in DOS or the BIOS it doesn't freeze, which makes me think the PSU (no-name old one) is just not powerful enough.

Leading this back to my question as far as the Corsair HX1200 though - I don't understand why it won't let this thing POST and this only further explains my confusion:

the ATX 20 pin connector has 4 pins carrying +5v, and per ATX spec each pin is rated for 6A max current, so 4 pins x 6A each = max of 24A +5v pull through that connector. The HX1200 has 30A on its 5v rail, so logically it seems impossible that the not-POST-ing would be indicative of a lack of adequate power, right?

And if that is the case... I have no idea why it is behaving like this. My other thought was that maybe somehow this thing needed -5V to boot, but I de-pinned the -5v wire from the old PSU and it still works just fine so I am out of ideas.

Hopefully there's a PSU guru out here who can help me solve this mystery and get the HX1200 to work with this thing, as it's all ready to go aside from not seeming to have stable power yet.

Thanks in advance everyone!