In the case of the B&W, the woofers are in parallel (4 ohms) and are 3 db less efficient than the 8 ohm midrange and tweeter; if a transistor amp is used it will have double the power into the 4 ohm portion of the impedance curve, thus driving the woofers with the 3 db extra needed for them to operate with the midrange and tweeter. The speaker cannot be driven properly by any amp that does not double power as impedance is halved.
Ralph, re "the speaker cannot be driven properly by any amp that does not double power as impedance is halved," you may recall that we discussed a similar point in another thread a couple of months ago. Wouldn't it be more correct to say that to be able to drive this impedance characteristic properly the amp has to have a low output impedance (relative to 4 ohms), and power capability into 4 ohms that is adequate to support the required volume? In other words, a solid state amp rated at say 200W into 8 ohms, but only 300W into 4 ohms, will do just as well with this speaker (IF all else is equal, and if the output impedance and 4 ohm output power criteria are met) as a solid state amp rated at 200W into 8 ohms and 400W into 4 ohms.
Regards,
-- Al