How to isloate turntable motor from making a hum


My turntable has an AC motor that is mounted right at the bottom of the turntable. The turntable itself is placed on top of the vacuum tube preamplifier. This placement makes the AC motor only half inch away from the pre-amp. Spinning at 33 amp produces a barely audible hum. But once it is switched to 45 RPM the hum level increases substantially. This morning I lifted the turntable up holding it about 10 inches above the pre-amp and the hum was gone! Once I lowered the turntable closer to the pre-amp the hum was gradually increasing reaching its max level once the turntable was placed right on top of it - when the distance between the motor and the pre-amp was shortest. Apparently, there is some form of interference going on between the turntable motor and the vacuum tube preamp.
Unfortunately, there is not enough space in my room to position the turntable to the side, next to the preamp. And I can’t raise the turntable 10 inches above it as it would make it hard to reach. I wonder if there is any isolation material I could use to place in-between the turntable and a preamp? Can an acrylic or plastic padding or a metal sheet used to block the interference?
esputnix
Shielding is really just a band-aid. Best to get some distance between components. Electromagnetic fields drop away quickly with distance. 
Russ, op says he can’t get TT and phono stage any further from each other. Maybe sell some of that expensive gear and get a bigger room. (Joke)

ps. I doubt the Steelhead is the source of the EMI. Steelhead has an outboard PS with a long umbilical. I assume the steelhead PS is out of the way on the floor or on a lower shelf.
"...Russ, op says he can’t get TT and phono stage any further from each other..."

Can't or won't?  I accept only certain compromises but in this case, it needs to be solved correctly. Turntables can be placed in many positions, they don't need to be on top of your gear.
Moving the turntable away from the preamp solved the hum issue. There is no hum regardless of the rpm - it is dead quiet now. While shuffling the gear components around, I've placed a monoblock power amp onto the preamp.  That made a preamp generate a different kind of hum. The bottom line, the preamp doesn't like any gear around, otherwise it picks an interference from a nearby component. It's best when it is positioned away from anything else, as far from turntable, a power supply or an amplifier as possible.