How to isloate turntable motor from making a hum

One thing vexing me this whole time is referring to the Manley Steelhead phono stage as a preamp. You would not be having these problems if this was a preamp. Your problems stem from the tremendous phono stage gain, something no preamp ever has or does.   

These distinctions are important because it helps a lot to understand this stuff. A phono stage includes RIAA equalization. Since the lowest bass on a record is -20dB the phono stage has to equalize this 20dB. That is a lot of amplification, and it comes on top of the 40 to 65dB gain needed simply because cartridge output is so low.

This is the real reason hum is so prevalent in situations like this. Once you understand it you would never in your life even think about putting something like a motor or amp on top of a phono stage. Preamp maybe. Phono stage, never.

If you look inside one of these things you will see the circuit is laid out or organized so that incoming power and transformers are kept as far away as possible from the amplification circuits. Usually both physical distance and physical shielding are used. This is the case with all components. But with phono stages it is taken to extremes, because the phono stage takes everything to the extreme. So it is worth it to call it what it is, and it is not a preamp. It is a phono stage.

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?

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.