I only have experience with the 6027 setup, but I suspect some of the common hardware has to be powered on regardless of how many nodes are online. When I was doing tests, a single 2x2680v2 node ran at every bit of 250w hashing with a 1050 onboard as well. Imagine my surprise when 4 nodes "only" pulled ~920w.
Upgrading to latest IPMI firmware and setting fans to "Optimized" mode might reduce power use (and noise too!) as well.
Yeah you seem to be correct, i'd imagine the backplate has a lot of electronics to provide 12v and 5v so it takes more idle power. Removing the Mezzanine sas / raid cards seemed to have dropped power consumption. Running 2 nodes at idle around 200watts now, so it's about 100 watts of overhead for the fans / backplate.
Haven't had time to setup xmrig yet, but i'll be happy if i can get it to 1200H/s @ 250-260watts 2 node server all-in.
This unit also came with 1U fans instead of 2U fans - I should have done my homework, but I assumed all 2U supermicros came with 80mm fans.
Need to figure out how to use ipmiutil to drop fan speed to 10%, these 1U howlers are LOUD at 50% idle. I might just pull all of them out and stick a household fan infront of the rack.
E5-2450L are a hidden gem @ $30 a cpu for dual cpu 650H/s. Finding hardware to run LGA 1356 is the only issue.