T7910 build

Heya, need some advice.
History: after 10yrs of running a successful T7600 build for intensive apps and gaming, the time has come where I need AVX2 CPU support. Dig around a little and T7910 looked like the best bet, similar size, feel etc.
So I get a mobo, 2xE5-2667v4, 6x16Gb RAM (2Rx4 PC4-2133P, only 6 because that's how many were left at a very good price); start replacing in the T7610 case.

Lo and behold, needs PDB...get a PDB. PDB cables don't match...get cables. The PSU (1300W) no match...get PSU. Cables for front I/O no match - at this point I went hands in the air and got a 7910 case with pdb, cables, i/o...the works (it even has the plastic foil on the front, go figure).

Assembly time! put everything together, leave 1 CPU just to check if it starts OK, RAM in slots 1-4, move "le old" CPU cooler and 1080Ti from the other system (I put it back together, works like a charm as usual) and liftoff! to a 2-7 amber light...

Now, looking at the RAM placement map I see that for 16gb stick I either put in all slots (8) for CPU0 or mix'n'match 4x16gb with 8x8gb. 

Am I assuming wrongly that it would start proper with only 1 stick of ram or 2 or 4?

NOTE: I enjoy tinkering hardware and this story spans 3months, mostly due to not enough free time but I kind of want to make it work.
Suggestions?