What Sensitivity with Tube Components?

Those speakers are extra hard to drive - inefficient - due to the small box and B&W trying to make more bass with it.

B&W Manual
88dB spl (2.83V, 1m) its not a straight 88db spl (1.00v, 1m) comparison.
So they are actually worse.
Do the windows have curtains ? if so have you tried closing them.
The manual also says they need to be at least .5 meters 1.6 feet from the back wall and side walls.
A Big TV between them makes it worse too - not sure if that is the case. 

Your amp makes 100 watts at 8 ohms. and only 120 watts at 6 ohms. Your speakers go down to 3 ohms to make their bass.
The speakers are making your amp struggle imo with the bass. This throws off the tonal balance. Makes the speaker therefore sound bright.

If you buy a SS amp make sure the specs show doubling going down - 100 wpc 8 ohms, 200 wpc 4 ohms.
if you buy a tube amp makes sure it has 4 ohm connectors on the back.
I would go with as many watts as you can afford which ever way you decide to go.
important.
try to bring the amp home for trial. if you can't do that bring your speakers to the store and listen there set up as close as possible to your home.

my 2 cents.

All makes sense but you are saying that these speakers should be connected to a 4 ohm output? The Yamaha R-S700 has an impedance switch on the rear which allows for 4-8 ohm speaker set ups, bi-wire & not. (see page 13 manual)
I have not tried this setting in the "Low" (4 ohm) setting but maybe it would be worth a try?
Also, These speaker are at least 1.5 fee from the rear wall but I've always felt speaker stands would help. (The wife is not into this as much as I am) So on the built in book case the remain!
As for tube, you suggest as many watts as I could afford. You can buy a Jolida 100 watt Tube for under $2,500.00. So price does not always seem to dictate great sound.
Do I consider Class A, B, AB Tube amplification?
Thanks.

Nothing between speaker, yes one side has heavy curtains which I have closed. Other side does not.

briefly looked at the manual. So you are using pure direct mode, cd direct amp ?When I said the 4ohm setting earlier I was implying for the tube amp. but this is an interesting amp.  

I have not tried this setting in the "Low" (4 ohm) setting but maybe it would be worth a try?

I am not familiar with that amp but would say - Yes.
See the wording here.  

http://hometheaterhifi.com/reviews/receiver-processor/receivers/yamaha-r-s700-stereo-receiver/

the R-S700 is bi-wire ready. In this case, it uses both sets of internal amplifiers (Speakers A & B) for one pair of bi-wired speakers. This implies a true bi-wired scenario, where separate discrete amplifiers are used for the tweeter/mid-range and woofer section of your bi-wire-able speakers.
Note that if you do use this bi-wiring feature, it is important to select the proper impedance (high or low) setting for the receiver. The high setting allows bi-wiring speakers with 6 Ohm impedance or higher, while the low setting allows bi-wiring of speakers rated 4 Ohms and higher. Similarly, the high/low impedance switch needs to be properly set for standard wiring too.

As far as wattage - B&W benefit from more good wattage.   I would try the above settings first and see how you like it. I believe you will get more oomph - and it will be a warmer sound.    

Do I consider Class A, B, AB Tube amplification?

That can become a long discussion.  there are real amp experts here that can help you out more.
Push Pull a/b tube amp works well from my own experience with lower efficiency bass needs.  And yes stands would improve things alot - but thats a no due to your wife saying so.    

Indeed!
Thanks for the input.