Tried the new SDRSourceEttus.dll with SDR-Radio V3, 64-bit, 2018-04-30_2059. It successfully displays rates up to 56MHz now. While 28MHz works smoothly, I find that the 56MHz setting has substantial "shuddering" in the audio (like the USB3 bus isn't keeping up), but it at least detects and negotiates.
Is there any way to select clock rates/bandwidths between 28 and 56MHz (ie, 40 or 50MHz), or are we stuck with 1/(2^n) fractions of the 56MHz max rate on the Bxx? Seeing as the old DLL maxed out at 16MHz (and 1/(2^n) fractions thereof), I thought it might be possible, but haven't explored the UHD driver sufficiently yet.
I've attached a screenshot of the new dialog and a logfile in case it helps.