Quattro Differential Confusion

A little over a week ago one of the front tires on my 98 A4 2.8 got a large bubble in the middle of the tread and I stopped at the nearest tire shop to replace it. The guys there happily put two knew H-rated Cooper Lifeliners onto the front wheels. My other two tires were some bottom of the line junk, so I figured I'd go out and buy some nice Bridgestone Potenzas for my new car about a week later. When I told the guys at the Firestone shop I wanted two tires they told me AWD drive cars should have 4 of the same tire, but since my other two were very new it should be fine. They said the reason for this is because it may damage the differential. After they said this I thought about those lousy rear tires I drove around with for about a week, and now I notice this strange humming noise from 30-40mph, especially when slowing down. It's a low-pitched sort of "woooo-wooo" noise that seems to be related to the speed/rotation of the wheels. I dont know if I'm just paranoid or what, but I was wondering if anyone might know the reason for this, and also the cost for parts/installation of the parts I may need.

Thanks,
Nick