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
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