Long RCA interconnects

You might be lucky. I used to have a 30’ run single ended from my preamp to mono blocks. With it running below the floor in the crawl space I used to have no issues. I then moved temporarily to a rental and had to run it on the floor and it was unacceptably noisy so I had to switch to balanced which I’ve been with ever since.  You also want to make sure the output stage of your phono stage is up to driving a long cable (as cable LCR will increase with length) - what stage are you using? 

Eastern Electric Minimax phono stage.

I recommend Blue Jeans cable LC-1. Shielded. Low capacitance. Affordable. I have several long runs. No loss of highs. No hum. No noise of any kind. Money back guarantee. 

Try running the AC lines from the same spot as the pre or amp, to reduce potential for hum. Ie, the same ground potential would be the result of the AC is from the same exact receptacle.

If you get some hum, try the ’same AC origin point’ trick. Play with the AC to try and reduce hum, is the deal.

We've made 6.5-7M RCA cables for people -- totally unshielded btw. No hum. That's a $35k pair of cables. Better hope there's no hum.....

Wow that’s a low gain phono stage (42dB/57dB) - so you could be in for some issues if you’re driving a long run off it. The stage itself is capable of driving a healthy 2-20V output but it would be good to know what’s coming in, what cartridge do you have? Hopefully MM or high output MC