Tires: wierd tire vibration, bad wheel bearing, tire vibration

Question
Hi, we just had four new 15 inch all season tires put on our 2002 Toyota Sienna.  From the minute we drove away we discovered that when we drive between 25-30mph, we get a loud vibration (sounds like we have the bass turned up really high in the car, but we don't) that goes away the minute we go 31mph or faster.  We took it back to the shop, and they drove it and put it on the lift and couldn't find anything.  Have you ever heard of this problem?  They thought perhaps a bad wheel bearing, but the description I have read of those sounds is very different.  This is like a low, but loud, hum.

Answer
O boy.  This could be a lot of things.
It could be a bearing.  If I understand your question right, this sound started after the new tires where put on.  In that case, it would be hard to believe it is a bearing, unless it just went out.
I would recommend that you have the tires re-balanced.  Are these tires on correct?  Some tires can only turn one way.  Are the tires the correct tire size?  Are they the correct OEM size?
If not, then there is your problem.
John