Tube Preamps

Only trying a tube pre will tell. Time to visit a dealer and get a loaner. As you have done, only putting something in your chain will tell. Try to get two different loaner's, that way you can hear if it is 'tube' you are hearing as opposed to a just a single unit.

Yes, your speakers are a tad bright and it's very possible for an amp to sound bright too... and pretty likely if it's solid-state. Some sound brighter than others. Yes, flat response is the goal, but you probably wouldn't like it if you heard it anyway.

A tube preamp could help if mellow-sounding tubes are used, but the bass will be less deep and distinct. I'm sure the tone controls on the NAD amp help out... if it has a preamp output maybe you could try the preamp output into the Rotel amp's inputs. That way you could simply turn down the treble. Do you have a nice padded rug on the floor???

Good Luck

Yes, in general a tube preamp, or tubes somewhere in the chain will reduce brightness or glare. At least that has been my experience. Something else I do that tames my speakers is I cut some felt to place around the tweeter...so it sort of looks like a bagel with nothing touching the dome itself......I can't expalin why, but that helps eliminate some of the edge in my room....I don't have an extremely expensive set up, but it is comparable to yours. Good luck. The felt makes my system less aggresive without a significant side effect.

I don't mean to ask the obvious but if you like the sound of your NAD integrated why not stick with that or go foe NAD's separates?

Plato said, "Yes, flat response is the goal, but you probably wouldn't like it if you heard it anyway."

An audiophile paradox from the philosopher Plato.

So if the goal is a flat freq response, why then would it not be what we want.

Bob