On further tweaking, it seems that some careful balancing has to be done to preserve the gain structure when used in different modes:
As I use Roon to stream to my Neumann lounge system, I tend to prefer using the volume adjustment via DSP as the DX5 is not within reach. This means that the gain levels have to be adjusted in the Neumann control app as otherwise, when I switch over to headphone listening, the volume is too low, as I need to knock back the DSP level in Roon to stop music blasting out at ridiculous, ear-bleeding levels.
So, knock back the Neumann control level, keep the Roon DSP level fairly high, possibly turn on high gain also depending on which cans I'm using (currently swap between my DCA Aeon, Truthear x Crinacle and my old faithful Beyer DT770s).
It's a bit of a faff, but means that I'm now able to free up a Raspberry Pi, a Topping D10s and a Hidizs S9 Pro, all the associated cabling and power feed.
Not tried the BT streaming yet, but it will enable me to watch the tv through my Neumann rig, unless there's too much of a delay.