Nola contender tweeters - modify?

safebelayer, May I suggest a Bijar? The iron rug of Iran.

I cannot imagine, having owned 4 or 5 models of Nola speakers, that tweeter harshness is likely.
I have Contender 1s, and I’ve never heard them sound like that - except for when they were first breaking in (weeks of continuous play, which you would have achieved by now.
You didn’t say which version they are: I, II, or III. And are they new? Because new, they can CERTAINLY sound a bit bright. Otherwise....rather unlikely.
I agree it with the other poster, that it  would be something over than the tweeter, such as room acoustics. I doubt there would be an electrical mismatch between the VAC and the Nolas.

I have considered that as a possibility yet haven't done anything about it to date. Good suggestion, thank you.

This sounds more like rom acoustics, or lack thereof. 

Try throwing some blankets/pillows around on the floor behind and between the speakers. I find this often greatly helps with treble hash. 

But your whole room is important.