What Sensitivity with Tube Components?

wcc10,
You have received some very good responses here. Al gave you the definitive answer,= your speaker is designed to be driven by a transistor amplifier. Small, inefficient and striving to produce bass(via low impedance). I'm a tube guy but tubes  usually won't match well with this type of speaker design. As has been said, tubes work better with higher impedances with a relatively flat impedance range. If you ever decide to try a tube power amplifier, get speakers with friendly  and flat  impedances. With your speakers, stick with solid state amplifiers.
Charles,

Yup, that is the message I'm getting. Makes total sense and I'm glad I posted before I took the plunge into tube amplification. I've had these B&W speakers for about 1 year and I always intended to "upgrade" at some point. This discussion has shown me that upgrading my speakers with a view toward going over to tube amplification requires serious homework before making that switch. It also shows me that where I buy is as important as what I buy. If the guy selling me the gear has no clue about what he is selling then chances are I will end up not being happy.
Mcintosh MC275 MK6 - B&W CM6 S2 don't match up very well as proved to me first hand.

An update; I did try the Bi-Wire set up with my speakers & Amp. With the Impedance switch set to Low and using two sets of 12 GA cables there was a bit more "oomph" but also seemed to be a bit of distortion again in mid-range. Switching back to impedance setting High cleaned up the distortion and sounded more like the cables single end at the amp and Bi-Wire at the speakers. I was using two different brand of cables so that must be taken into consideration but I could not justify using 2 cables as there was not a great deal of improvement. Speaker placement is highly important and I guess right now I'm going to have to be satisfied with what I have. Which is not all that bad considering.
Thanks to all for the help!

Also i would look for a front ported speaker if in enclosed bookshelf !

Good point.