What Sensitivity with Tube Components?

I would not try to cure the brightness by going to a tube amp.  Somewhat counter-intuitively, it would stand a very good chance of worsening the brightness.  The reason is that relatively small speakers such as yours, that strive for good bass extension relative to their size, are often designed with a lower impedance in the bass region than at higher frequencies.  The idea being to capitalize on the fact that a solid state amp will deliver proportionately more power into low impedances than into higher impedances (assuming the amp is operated within its capabilities), thereby enhancing the speaker’s deep bass response.  A tube amp will not perform in that manner, and depending on a number of variables specific to the particular tube amp and speakers, and also on which output tap is used, will very often deliver even less power into a lower impedance than into a higher impedance, resulting in an overly bright sound with a speaker designed in that manner.

I couldn’t find an impedance vs. frequency curve for your particular speakers, but that is one of the reasons that many B&W speakers are not considered to be tube friendly.

Good luck.  Regards,
-- Al