Tube Preamps

This abstruse hobby of audio cannot be logically explained. Much of "high-end" audio is based on psycho-acoustics. If the the soound is liked, stick with it. In the late '90's I opted for tubes in the form of a Melos Sha-Gold. It sounded great at the time. At present, I listen to Sennheiser HD-580's via an aged Adcom GCD-700 and Creek OBH-11. It also sounds great. It's all subjective.

DON'T CHANGE ANYTHING! Get a powerline conditioner first. If you do not currently have one, borrow one. It may solve the problem.
I really have to say cables and powerline conditioners are the audiophiles only tone controls anymore. The powerline conditioner would smooth out your top end. (it may do some stuff you don't want, but the right pwer conditioner will do pretty much want you want.)
I use somewhere in my system: Monster AV2000, Monster 7000SS, Adcom AC (I have THREE of these) A Rotel AC in a box, a 50' extension cord conditioner, various homemade AC cords, some aftermarket AC cords.. Various Ferrite clamps..
And plan on buying an AC regenerator soon.

I second Rwwear's suggestion of experimenting with various toe-in angles, and I suggest that you do that as thoroughly as possible, trying many angles. Here's my reasoning:

Bob made an excellent find with the S&V frequency response plots. Here is a direct link, which provides additional information:

http://www.soundandvisionmag.com/speakers/2271/test-bench-b-w-cm-series-home-theater-speaker-system.html

As you'll see there, the plot reflects an average over a +/-30 degree listening angle, with double-weight given to 30 degrees (presumably corresponding to a typical listening angle with the speakers aimed straight ahead). The corresponding plot (the green line in the figure) is basically a roller-coaster in the upper mid-range, swinging nearly 10db peak-to-peak.

As Atmasphere (Ralph K.) has pointed out in other threads here, it is common for certain amplifier distortion products to create the subjective perception of brightness. My suspicion is that the reason for the different behavior with the two amplifiers is that with the Rotel the predominant distortion products happen to fall at the peaks of the roller-coaster with the music you tend to listen to, while with the NAD they are happening to fall into the trough.

But regardless of whether that theory is true, it seems to me that the roller-coaster frequency response curve, that occurs right in the problem area you are describing, is likely to be the root cause of the problem. And hopefully it can be improved by simply re-aiming the speakers. Many of the other suggestions that have been offered may subsequently help further, but without first addressing that frequency response roller-coaster they strike me as using a band-aid when a tourniquet is required.

Regards,
-- Al

Wow!! Great responses from everyone and they are greatly appreciated. I will try the different toe ins first, being they are the easiest and least expesnsive. I will post my findings.