Hyundai Repair: Hyundai Tiburon vibration, low profile tires, wheel vibration

Question
I recently got four new wheels and low profile tires for my 1997 Tiburon, and it begins to vibrate after about 45 mph. I took it back and they said they turned them on the hub (how can that help?) and rebalanced them. It was better, but it still vibrates between 55 and 65, after that if you can go that high of speed, it smoothes out. They had to put 1/8" spacers on the front wheels to accomodate the studs, and want me to bring it back and try spacers on the back wheels just in case. They also said it may be something that was happening before, and the low profile tires magnify it? Any ideas?  

Answer
If the vibration was not noticeable prior to the tires and wheels being put on the car, then the problem is most likely with the tires or wheels.  While low profile tires will transmit more terrain roughness than the original tires, vibration transmission on smooth pavement at highway speeds should be similar.  

There may be a problem with one or more of the tires being out of round.  While the tires and wheels can be balanced, it's impossible to overcome the physical issue of a tire being significantly out of round.

The wheels may not fit the hubs properly.  The center hole in the wheel should fit over the center part of the hub.  If it doesn't, the wheel will not fit squarely and will cause a vibration.  

It sounds like the shop is genuinely trying to rectify the issue, but I wonder how much knowledge they have regarding tire/wheel vibration issues.  All I have to go on is your description, so it's difficult for me to determine what they've done and checked.