How to isloate turntable motor from making a hum

Never a good idea to stack like that, but we gotta do what we gotta do. Electric motors make electric fields. Preamps are especially vulnerable, phono stages even more so. Distance is your best protection. If you can rearrange to move the preamp lower and farther away that will help even if stuff winds up being stacked simply because it will move the phono stage further away.  

Next thing you can do is shield. Vinylzone has one way with the copper sheet. Another is to shield the motor itself. Another is to shield the phono section, or even the whole preamp. Shielding can be anything conductive doesn't have to be copper sheet doesn't even have to be solid. Sheet metal is used inside a lot of preamps to shield the phono section. You can also try battery grounding strap, which is braided tinned copper. Battery strap is fairly cheap and can be cut up like fabric. You could also search for Faraday fabric, more expensive but a lot better looking.    

Keith Herron suggested aluminum foil as a cheap and easy test. Never tried it myself but, Keith Herron, worth a shot. 

Any of these whatever you try will work better grounded to earth ground- your AC ground will do fine. Just not grounded to the preamp.

What MC said.  Your situation is no surprise, given the extreme proximity of tt and preamp.  Cheap and dirty is to use heavy duty alu foil between them or around the motor or whatever works, but it should be grounded. For that you could use a clip lead from foil to ground. Could ground it to the ground lug on the Steelhead.  I own a Steelhead too; I think it has two grounds, one for audio and one for chassis.  Try each option.  I'd be interested to know how it works out for you.

Your motor is not well shielded. As others have suggested trying any grounded metal under the turntable might help as would a high output cartridge. It could also be the reverse. Fields from the Manley could be causing the cartridge to hum. Grado's are notorious for this.  In either event it should not happen. The phono stage is never that far from the turntable.  Mine is also directly under the turntable and the system is dead silent. You might try different grounding schemes. I had run a separate ground from the turntable chassis to the phono stage along with the ground wire from the tonearm. If I disconnect either I get a hum. 

You know now that I think of it, at the level of these components what I would do is first test with aluminum foil. Wrap the motor, and fold a sheet for between the table and preamp. Then if that works upgrade the whole thing with some Krissy mats. I have never thought of using them for shielding but that is basically what they are doing. Why settle for eliminating hum when you can get an upgrade out of it?

copper sheet covering tube preamp??
good luck