I find it a bit strange that you can't get the action required from your ML sub.
I have a complete dCS stack myself and I use the rca output from elgar+ to drive the sub and xlr to drive my mono blocks
The output signal both from xlr and rca output is equel, and I adjust the volum on the sub to my "taste", no problems.
On the Elgar+ there is a switch on the back to choose either max 2V output or 6V, however if it is load enough with 2V on the main speakers, no adjustment is required if you are using 2V as a default setting (don't adjust this setting during play back)
The volume control in front of Elgar and the remote is equel.
I use my system without a preamp, if a preamp is used I assume that the sub must be connected to the pre amp and not the elgar dac.