X670 is, literally,
two daisy-chained B650, all going through a
single PCIe 4.0 x4 link (Intel is now using
PCIe 4.0 x8 for its consumers chipsets). And X870 is a minor design update on X670—so minor that there's no urgency to offer the single chip as B850. This looks like a pitiful marketing trick to have users think they need to buy a new and expensive 800 motherboard with their Ryzen 9000.
See that X670E Gaming Plus block diagram above? On the first chipset, you already have a x4 M.2 slot, plus all the USB and SATA connectivity. Then on the second chipset you have a second M.2 slot, three PCIe slots (x4, x1 and 3.0 x1), the NIC and some more USB. Any network traffic is taking one lane out of the four lanes that the two M.2 drives and the three PCIe slots also need to reach the CPU. If you have your two audio cards in these PCIe slots, there are at the bottom of this chain, sharing traffic with the NIC and USB but it should still fit; now if just
one of these two PCIe 4.0 M.2 requires I/0 at the same time, there's not enough bandwith for everyone to operate at nominal rate.
X870 is going to be the same kind of oversubscribed contraption.
If latency matters, I think you should stay clear of X670/X870 and try to have only one hop to the CPU, hence a B chipset.
I dont want to bother casey on it atm as he has help me so much trying to get this going. I could send you my efi if u wanna give it a shot.
Open a support thread here, the more eyes on it the better. (I'm not an AMD expert by the way, I'm mostly follwing out of curiosity.)