$1,500 phono upgrade worth it?

That would rightly piss me off. I've no hum.

If when playing music at whatever level and sitting wherever you usually sit the hum is audible only when the music isn't playing, ie gets drowned out by groove noise, then no worries. 

Groove noise… that is a problem. Well, this is the benefit of moving up in analog rigs. A good TT / phono stage will have no groove noise or hum.

I remember at being shocked with my first good turntable that all the groove noise and ticks disappeared… and if there were a few ticks using a cleaning machine and they disappeared. Now, my collection’s background noise is completely absent. 

OP,

Please advise on what phono cable you are currently using from the tonearm to the phono stage. If it isn't a "dedicated" phono cable, that could easily be part (if not all) of the excessive noise issue you're experiencing. 

Thanks

What I can't figure out is the noticeable hum when I turn on the stage with nothing playing

I agree with @ghdprentice. The phono pre is very important and one at 3 times the cost of the present one will be a big upgrade IMO. However, if you have a ground loop problem which I suspect is the problem, the problem will still exist with a different phono pre.

As usual, @millercarbon  has given good advice on how to go about tracking and eliminating the problem. This should be your 1st priority. Then the new phono stage

Then there is the hiss you say is present. If you have the gain set too high and have high sensitivity speakers, you are more likely to have hiss. This is a good reason to list your system so that others can help troubleshoot the problems.

You can get a solid upgrade in sound for less.  The Lehmann Black Cube SE II is an excellent value at ~ $1200 or so.  Outboard power supply for low noise and excellent dynamics.