PrimaLuna EVO 300

This seems very odd. How big is your room? A pair of 212’s at 60% sounds like a lot! Are you sure you are using the low level input and not the LFE input on the back of each 212?

What happens if you use the built in headphones with the amp, does it still sound attenuated? 

And have you tried adjusting the phase of the amps, in case something is phasing out?

Last, have you tried using speakon connectors to the speaker outs instead of line outs?

I've tried everything you mention EXCEPT the headphones, and that is an excellent idea. Will try it later this week when work lets up. I think my room is just too small for these subs. I wanted end-game subs because they'll soon move to a new, acoustically treated listening room. But the room they currently occupy is only about 11 feet wide and 17 feet long with 10 feet ceilings. Not a lot of room for 4 active and 4 passive subs. They still sound amazing, but I was expecting a little more and I've had issues before with connecting subs to the PrimaLuna (using line level and sub outs). Thank you for your helpful response!

" The voltage on the sub outs is so low that no sub would do anything unless maxed out."

How did you arrive at that conclusion? Did tech support @ Prima Luna help you to make that diagnosis?

Verify your REL's setup with REL tech support. Positioning and recording used  is also important.

Something simple, likely amiss

I used a voltage booster and the subs I had then worked perfectly. Without the extra voltage, most subs would make zero sound until maxed out. This is when connected to the stereo RCA sub outs. When using the Neutrik cable (REL's Blue Cable), it was considerably better.  

Coincidentally, I own the Evo 300 power amp, REL 212/se and the Cornwall's British cousin (Tannoy Legacy Arden).

I tried both the RCA connection from my McIntosh c2300 preamp, and the line level connection from the Evo 300. While I did notice the RCA input required more gain from the 212/se, the stock line level neutrik connection only required about 1/4 travel of the 212s gain dial. If I go any higher (say to 1/2) its way too loud and overpowers the mains. 13'x17'x16' sloped ceiling.