I have a LOT of experience with USB converter and DAC's, having owned and heard quite a few.
The best is the Off-Ramp, then the Audiophellio, with the Audiophellio being the better value, but there is zero doubt the Off-Ramp is clearly and obviously better.
I have owned a WFS and fed by its internal USB it was pretty ordinary I have to say - cold, analytical with slight sibilance issues, but fed by the Off-Ramp virtually all the issues gone - still on the analytical side for my tastes but to the point I would say its a personal preference thing - and the detail can be a bit overpowering - but a very good DAC that way.
Still if you are looking to spend the money to match that sort of system I would look at the two best DAC's I have ever heard - a Playback Designs via DSD (its not as good via PCM - but via DSD its supreme) and the Phasure DAC via PCM.
Neither requires an external converter - the internal converters of both are top notch.
Thanks
Bill