I will be installing Kahrs or Lauzon engineered over plywood subfloor over radiant heat. The application is on the second floor. We want as much sound deadening as possible. What are the underlayment choices and brands? Any opinions comparing the two floor companies? Will float either NextStep/Lauzon or Cottage & Castle/Kahrs.
Thank you in advance.

If is on the second floor and over plywood with radiant heating, I would suggest not using an underlayment. It will be much more solid without it. If you are set on using some then cork would be best.

Kahr's will only warrantee their product over their own underlay. I don't know about the other product