Core i7 5820k, 4x8GB Corsair Vengeance LPX 2400, Noctua NH-D15, Firepro W7000, Seasonic P-660, 2x 850 EVO (500GB and 1TB), 1x WD Black 4TB, 1x WD Black 2TB.
I've tried to connect an USB card reader (internal header, USB 2), two front USB 3 and one USB 2 headers (case), USB keyboard, mouse, USB cables to an UPS, a Cintiq, a USB hub on a Dell monitor, a printer, and two USB 3 external HDDs.
I've removed everything from the mobo and I get the B4 code without anything (including the internal SSDs and HDDs) attached.
The Firepro worked (and still works) on three different mobos, and I've reassembled the old PC with the PSU (new for this build) and all USB devices listed, and I have no problem.