Kirby vs Royal airflow
Ben,
I am not sure what is going on with your Kirby's but the sound of the motor in my D50 Does Decrease in pitch when the nozzle is lowered to proper cleaning height just as you would expect. My D50 Does have a new Amodel polycarbonate fan in it but I don't think that is what's causing the difference in pitch.
Now Kenkart is only partially correct when it comes to Kirby vs Royal airflow and here's why:
It all boils down to Nozzle and Rug Plate design.
He is correct in that all Royals do not seal to the carpet because if you look at a Royal rug plate you will see a notch formed into it on each side which allows airflow to enter on each side when the nozzle is lowered down to the carpet preventing a complete seal from taking place.
Now on the older Kirby's. like my D50, you will not find any notches in the rug plate therefore an older Kirby will form a seal to the carpet when lowered into cleaning position.
I am not sure when Kirby decided to add these notches or indentations to the rug plate but my Heritage II Legend and both my G3 and Sentria have the notches in their rug plates so that means the newer Kirby's, just like Royals, do not completely seal to the carpet when the nozzle is lowered to the correct cleaning position.
Now after all that, I don't think nozzle and rug plate design are causing your problem either because my D50 acts just like my Royal 8300 and all my other Kirby's. You might want to check your brushroll for wear and make sure it is adjusted for proper bristle length past the rug plate.