@brian @robdarling
Feedback from Tony W at Bluesound (below). Good news (for me) is that even with the 48KHz conversion things seem to be working well here - I can group any combination as long as I leave out the Raspberry Pi running DietPi. It’s intriguing, though, that the RPi/DietPi only causes a problem when one or more Bluesound speakers are in the group.
So, can relax knowing that Bluesound and Roon are looking at solutions and spend some time over the weekend trying out the DSP / Room Correction features in 1.3. I’m impressed so far with 1.3 - particularly the speed of operation (doing background work using 4 cores yet still plays music without any problem) and also the speed of reconnection of the new iOS app - that’s a big and welcome improvement 
This just leaves the important question of why the Pulse Mini, Pulse Flex and Pulse Soundbar have a setting which appears to force ROON to convert to 48KHz but the Node 2 can accept up to 192KHz from Roon.
I have forwarded this information to our BluOS Lab and they have confirmed this is one of the things we are looking at…
I unfortunately have nothing further to report until they get back to us but will have something as soon as possible as it is a priority for us.
Thanks for your patience
TonyW