$1,500 phono upgrade worth it?

Tweeter hiss is common for high eff speaks. 
 

 Tone controls? Turn down 2 notches treble. 
 

 Unplug wires, wipe off,  reconnect, make sure all connections are snug, 

check for touching cables as well.

clean the records well.

It is possible to have a system with high efficiency speakers with no hiss or hum. I have a tube pre-amp, tube amps, and horn speakers. I have no hum or hiss when using my system. It is dead quiet. I cannot hear anything when my turntable is running before placing the needle on the vinyl. I do have everything plugged into one power conditioner. I did replace the stock turntable power supply with an SBooster Linear power supply. 

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.

The only time I had hum was when I had a speaker cable that failed at the connection. It got bent when I moved my amp and the solder failed at the speaker connector. I noticed the hum immediately since it was so unusual. 
 

Perhaps it’s a cable from the turntable to the MC stage or to the integrated amp? Is it possible to use a different set of cables between these components? Also, that MC stage does show a ground connection for the turntable. If the MC stage uses a wall-wart for power that could be causing the noise. Those are notoriously noisy.