probably just the cheap tires
could be the wheel bearings, but
since you noticed the hum after the tire change i would say its the cheap tires
i have cheapo tires on my honda civic, with a hum, but no steering wheel shimmy, so i figure its the cheapo tires
do you notice the steering wheel shimmy at high speeds?
my fix for this is make your exhaust louder
then you wont hear the hum

buying another cheap tire probly wont fix the hum, only make it louder.