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You can observe the cartridge output on an oscilloscope to see how much you have. You can also measure the amount of signal you need to put into the amplifier to get full output. These measurements will tell you all you need to know, since the power output and gain are determined by the tube and supply voltage and such. You can get a trifle more gain by bypassing the cathode resistor.
I do not recommend a solid state preamplifier. You are going to have impedance and power supply issues. What tube is in that unit? You have a chassis that could accept a second tube socket so you can get all the gain you want.
It's highly likely that the dropping resistor for the heater of the tube is the motor. That does limit your selection of tubes, but not all that much.
The mechanical hum is likely due to insufficient lubrication or worn bearings or a hardened idler wheel.
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