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.