Experts: Why is preamp important?

"and least understood- control the interconnect cable between amp and preamp to minimize the effect of the cable. Most sources cannot do this BTW."

I know this isn't to topic but Atmasphere's #4 purpose is so intriguing that, like Don_s, I would appreciate it if (with Jeffkad's indulgence) someone could expand on this point.

the B&W's require very competent amplification. given your $3k budget it would seem that you would want an integrated amp from "you know who"- check the listings under int.amp-SS.
$3500 would get you a component that you could be happy with for quite a long time- powerful and solidly built. OTOH, a separate preamp and pwr amp would require a very good pair of (balanced) interconnects between them- perhaps costing $500 or more.

Don_s, If a preamp has a low output impedance, many of the differences you are accustomed to hearing in interconnect cables go away.

In fact the balanced line system used for decades in the recording studio and broadcast was conceived to eliminate (quite successfully) interconnect cable interactions.

Many sources do not have particularly low output impedances. For that reason it is useful to have a preamp line stage to buffer those impedances from the interconnect cable. This prevents the cable to the amp from having any significant effect on the sound.

Quite a few preamp manufacturers do not recognize this function of preamps, so there are many preamps out there wherein the interconnect cable between the amp and preamp is quite audible. So if you hear big effects between interconnect cables between the preamp and amp, then your preamp does not support this function.

Having just recently going from an Outlaw Model 990 home theater processor to a Pioneer VSX-74TXVi which is similar to your Pioneer receiver which I am using as a home theater processor, I would say the preamp is very important. At first I didn't think going from a separate processor back to a receiver would be a big upgrade but in my case it was. Most of the upgrade in sound was probably due to the more sophisticated acoustic measurements with the Pioneer vs. the basic speaker level adjustments with the Outlaw. But also having the iLink ability with my Marantz DV-9600 to capitalize on the jitter-free transfer system helped as well. I am using a Parasound 5 channel amp to power my Magnepan speakers and I can say that the separate power amp helps with the sound as well. Wider soundstage, more detail and better dynamics vs. the Pioneer's power amps. If you ask me which has the greater impact in the sound quality, I would say the preamp in my case. Using the Pioneer's MCACC mic system is several levels higher than the Outlaw's basic mic system. But there are positive audible differences going to a separate power amp with your Pioneer as well.