Thanks for the replies, to answer the above...
I've got a few plugins installed, I've tried disabling them, restarting and the issue persisted, it didn't seem to matter which ones were enabled or disabled to replicate the problem.
Also tried choosing different outputs, like SPDIF and Headphones but it still just occurs in WASAPI exclusive mode. Just checked WASAPI Shared mode and that works fine.
I can't see version 3.5.8501 publicly linked on forum, is it a beta/developer version?
Upon further testing I've downloaded a clean portable version alongside my portable version and cannot replicate the issue even after duplicating the settings so am having a look into it when I get some more free time and will report back if I find anything out...