Tube or not to tube?

You'll be moving in a different direction. Perhaps up. Why not purchase one of the used Prologue 2 integrateds that come up for sale and try it in your system. If you don't care for it, you'll be able to resell easily.

Your speakers will be a determining factor, however. IMO, anything below 89db/6 ohm minimum will best be powered with 150+ wpc solid state, and not a tube integrated amp.

Check out the Cayin line. Incredible build quality and looks with magical sound to match. It should be a very dramatic improvement over your NAD given you have ample power.

A move up. No question.