First this problem appeared only on one machine that was challenging for other reasons, and the other v 1.4.0 installs went fine.
Did an initial 1.4.0 upgrade on my Neal-Machine Win7 monster, including the updates to the 6300 (as it was in the operative position and I didn't want to waste time switching to the 6700). Everything worked awesome.
Where I confused the configurations at SmartSDR v1.4.0 was on the iMac running Parallels v9 & Win7.1 as the initial audio devices after install was invalid. They were pointing to DAX reserved channel 8, which is of course invalid for any use with a 6300. DAX and SmartSDR both hung, stuttered and froze alternatingly as a result, with full crash followings. Entire Virtual Machine needed a reboot to clear.
Took a couple runs at reconfiguring the Windows Sound Devices Defaults until the USB dongle to an extra set of David Clark headsets came alive. Everything rock solid after that was done.
Thinking out loud some of the settings could have been legacy from prior versions of SmartSDR running the 6700, or something in the "user error" category, but it would be useful to include some setup notes for anyone whose install doesn't given them clear clean I/o audio routing. The User Manual was offline so perhaps it is already there?