Mapman, Bob may be 'right' as Tvad sez, but the ratio is way beyond any nominal ratio you see bantered around like 10 to 1 for example. In this case it would be very close to 80 to 1.
Personally I'd tend to trust ARC when it sez a 20K imput impedence would be adequate (I actually did that without regret) and not get my panties in a knot over what the difference would be between say 20K and 47K or even 100K. I'd even take bets that in this instance even Bob couldn't tell the difference if he was listening with a stethoscope. In fact I'd take bets that it would be unlikely that he would hear any meaningful difference if the amp's input impedence was as low as 10K.
Personally I'd believe ARC. I'd even go so far as to think that their recommendation would be conservative and act accordingly, spending the copious amount of time necessary to solve these issues to insuring that the amps output impedence was a good match for the speakers to be used, to finding the right tubes to flesh out the pre-amp's tonal issues when matching it with any particular amp, or just sitting back and listening to some good music.
But hey, thats just me. I'm not very anal about all of these techie issues. :-)
