I agree that the guidance on choosing the impedance/crossover point is confusing. I did some extensive experimentation with my own Vandy crossovers, and also helped a friend set up his 2Wq subwoofer, and the results yielded more variation than I expected.
Setting the impedance/crossover point to one level below the amplifier's input impedance seems to "fatten up" the overall bass response. However, the exact results were not the same for the several different amps involved. My own system has a Bryston 4B-ST amp, and in this instance I did eventually set the crossover to exactly match the 50K ohm impedance of this amp. I did not care for the tonal balance when using the next lower crossover setting. (Interestingly, John Rutan of Audio Connection had advised me that Bryston amps tended to be a little "squirelly" in this regard, and John suggested trying a setting one step above the input impedance, a setting that matched the input impedance, and a setting one step below the input impedance.)
With my friend's system, which uses Adcom power amps, we found the best setting overall was one step below the 50K ohm input impedance. Setting the crossover at the 50K ohm level resulted in bass that was too lean, and a mid-range balance that was a bit "off" (sorry for the technical terms like "fatten up" and "off", but they're what come to mind).
To get a specific answer to your question about why one would set the crossover point at one level below the input impedance of the amp, you may have to contact Richard Vandersteen for more info.
Setting the impedance/crossover point to one level below the amplifier's input impedance seems to "fatten up" the overall bass response. However, the exact results were not the same for the several different amps involved. My own system has a Bryston 4B-ST amp, and in this instance I did eventually set the crossover to exactly match the 50K ohm impedance of this amp. I did not care for the tonal balance when using the next lower crossover setting. (Interestingly, John Rutan of Audio Connection had advised me that Bryston amps tended to be a little "squirelly" in this regard, and John suggested trying a setting one step above the input impedance, a setting that matched the input impedance, and a setting one step below the input impedance.)
With my friend's system, which uses Adcom power amps, we found the best setting overall was one step below the 50K ohm input impedance. Setting the crossover at the 50K ohm level resulted in bass that was too lean, and a mid-range balance that was a bit "off" (sorry for the technical terms like "fatten up" and "off", but they're what come to mind).
To get a specific answer to your question about why one would set the crossover point at one level below the input impedance of the amp, you may have to contact Richard Vandersteen for more info.