How Much Power Do Vandersteen 5a's Really Need?

yes, the more power the better they sound.....also (very important), you must have a high pass filter properly adjusted for your amplifier, or you can damage the speakers.   Don't just plug them in and play. 
Download the owners setup manual, and observe all they say.

pick the right tool for the job.

Broad sweeping statement, but I’ll bite.

Well designed horns with high quality supporting components can be every bit as articulate, detailed and refined as -insert your brand of low sensitivity loudspeaker here-. And where they shine over -again, your brand of loudspeaker- is in dynamics.

So two tools for the same job.

Oz

A mere 100 Watts on a 5A is a waste of a good speaker. 

Fair to say I'm a bit baffled by the widely divergent opinions on the amount of power necessary. 

I do have a question that has me completely confused, though. It was explained to me, by the current owner, that you connect your preamp via interconnect cable, directly to the external crossovers/high pass filters, which then connect directly to your amplifier.

While a very unusual connection scheme, I can understand how that works. What I cannot come up with any answer for, nor can any of the three audiophile friends I posed it to, is, how do the internal amplifiers that drive the low end get a signal if the entire signal from the preamp is going into the external crossovers/high pass filters? We're lost in that regard. 

From vandersteen:  "This top of the line High Pass Filter is specifically designed to be inline in front of your amp to roll of frequencies below 100 Hz. which are passed to the Model 7 series speakers and reconstructed in the amplifier powering the bass driver."

So the bass amplifier knows the filter steepness of the HP filter and corrects for it.  Because the filter is a gentle rolloff (I think they use a simple first order curve), you're still getting signal below 20hz and it's just amplified right back up by the bass section's amp.