So! some limited success.

I've discovered that the DSP33 speaker I chose to put to one side is actually happy to be master and will accept volume control. and it'll control the other two as slaves as well.

Not put all three together yet, but both other speakers will accept being the slave right when this particular speaker is master left.

So yay, that's an advance, but why on earth would the other two DSP33 speakers not accept being master? what's different about them?

David

Lounge: LG 55B6v, Some Denon receiver, Anthony Gallo Reference AV mains, Totem Dreamcatcher centre, SVS PC-4000, AppleTV4K
Office: Audioquest Dragonfly, Cyrus 6DAC, Monitor Audio CPW Gold
Headphones: NuForce HDP, Audeze LCD-2, Sennheiser HD6XX, B&O H6mk2