Hi Fabien;
The question of which sensitivity setting is 'best' can sometimes seem confusing. I will try to clarify: In theory, you don't need gain in the amplifier if it can't be used to provide undistorted output. In other words, if the sensitivity on your amp is such that the signal would be clipping horribly at full volume control rotation, it's too high for the installation. Also, the amp will be amplifiying more preamp noise than is necessary at normal volume levels. Bryston tries to avoid this situation by providing different sensitivity settings in our amplifiers
In the example you gave above, the 9B SST can be set so that it reaches full rated power, (140Wpc), at 1.1V, 2.2V or 4.4V. Assuming your preamp provides 3.0V at full volume control with your present signal source, your choice would be the 2.2V setting. That way, you would have the capability of reaching full rated power if desired, but still keep the noise in the system as low as possible.
Some people prefer a setup where the volume control increases rapidly, and reaches earsplitting volume at only half the volume-control rotation. I tend to think it's safer and more noise-free overall to set the system up, if possible, so that the amp won't clip until almost all the control rotation is reached. That's what Bryston recommends.
On your second question, a passive biamped setup actually provides almost the exact same power overall per driver, since both amps receive the full spectrum of music. If you detect an improvement, it may be because there is lower distortion in the cables with a passive biamped setup. Much of the same improvement can be obtained through bi-wiring.
I hope the above is helpful, but please let me know if you have other questions. Thanks for choosing Bryston!
Chris Russell
Bryston Ltd.
