$1,500 phono upgrade worth it?

I'd be exasperated, and suspect of that unit as you are.

Phono, even without a problem, it's a quest and luck, I believe in return option.

I would select a phono stage I could return, on the expectation that the noise will disappear. If noise is still there, return it, figure it out first, then upgrade after listening to existing without noise for a while.

best of luck with it,

Elliott

femoore1 and I agree,

tube system, highly efficient horn speakers, tweeters just juiced up, SUT, phono, no hiss or hum, dead quiet until the needle drops. My tonearm wire needed replacing, it was causing problems because of a design flaw, insulation wore off, wire got bare. Fixed, back to zero noise. I've had poor connections to cartridge pins, tighten, noise gone.

I just listened to Sgt. Pepper's, reel to reel, there a slight bit of tape hiss between tracks, but that's from the tape, S/N ratio of tape, not from the system. OMG was it awesome, 1st time I have played it since I made upgrades hither and thither this past year.

my friend has always accepted a slight bit of hum from his tube amps, far less efficient speakers than my horns. I thought it might have been a problem with the cartridge/tonearm we just worked on, but it is 'just there' on other inputs. I just asked him if he wants to go thru the whole system, contact cleaner, tube tester, fresh connections everywhere, including cleaning, perhaps tightening tube socket pins if needed.

He did have to send his custom tube amps to the maker, someone has messed about with them before he bought them used, and the maker had a few changes of his own. Hum was gone, for a while. 

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but it does seem OP's phono stage is suspect, his system is quiet on other inputs.