I agree that the Boss WL-20 had the cleanest tone of the wireless I've used.
Downsides for me were all practical live use ones:
- 2.4 GHz and consequent noticeable drop outs (key deal breaker for me);
- 'rigid' form factor was not so good eg with Ibanez SR basses;
- my older ones had a not great battery life of around 3 to 4 hours, if I remember correctly.
The NUX trumps it on all three counts, plus the handy charge case, so whilst I would have loved to have held onto the Boss, it was a no brainer for me to switch.
I generally use a cable at home which is where nuances of tone are going to be most prominent - in a live mix I wouldn't be able to notice it, as you have alluded. Besides, I'm too preoccupied trying to get the notes right!