Dedicated 20 amp lines/should i use a sub panel

I ran 2 x 20amp runs on 10 gauge from 2 side by side breakers for monoblocs. I ended up not using one of them because of a hum loop. Let us know how you get on.

Jim, Some replies below.

By side by side, do you mean the breakers are installed directly across from one another? Connected to the same main bus breaker tie. (Both are fed from the same leg, Line.) Or, do you mean one is installed above the other, side by side? One fed from leg, Line 1 and the other fed from Leg, Line 2?
>>> Same leg/busbar. One 20A breaker above other.

Did you use Romex?
>>> Yes. 8/2 NM-B (not 10 as I said before) cable from Lowes.

Did you try to keep the 2 cables separated from one another at least 6" after getting them out of the electrical panel, as soon as practical? Especially a long parallel run?
>>>Yes. 12 feet apart.

Is it possible one of the 2 is running parallel next to some other branch circuit?
>>>It’s possible but not obvious.

>>>I also tried tying the ground wires to the same terminal, removing the ground (to test only) and moving other equipment to different runs to no avail. I’ll be honest, I abandoned messing around with it after an hour and removed one leg and used the cable for another project. The one 8/2 run on a 20A breaker into a 20A L5-20/L5-20P receptacle/plug sounds great.

Jim,
Thanks. Correct - that’s how I received it from the seller. Not sure if Stu R. originally did that or if it was the seller. As the REL is fine and grounded, the hum loop may be from the monos. I dunno. The Croft preamp has the safety ground intact as factory supplied. The 401 is also grounded. Interesting notes on the steel plate. However, I’ll definitely try it on the DIY 4 duplex receptacle unit if I can buy one in that size. Thanks for your help and apologies to the OP for going a little off-topic.

Jim,
8/2 with ground. You are correct about the legs. I also tried it on both legs. So instead of 22, 24, where I originally connected it, I had tried it on 23, 24 but it still hummed. In fact, as it’s a basement space, I just ran in a 12/2 on 15A breaker on the same leg to see if it still hummed with the (2 wire) monos. It did. But it doesn’t when I add my REL sub. So all is not lost! I should say that there is normally no hum from the rig.

Looks like I should expedite my plan to rewire the power cords on the power amps with new OFC cables including connecting the safety ground. I’ve been meaning to do it after changing the power cord on the preamp with very positive results. Thank you.

Thanks @almarg. Yeah, I'll try it with and without the safety ground and see what sounds better. Thanks for your vote of confidence in the amps' wiring! The Speakon black goes to the right speaker ground IIRC. Not sure I'll bother with the second run. It's fine and juicy as it is and I've other fish to fry (like a new idler wheel from AF for my 401!)

Welcome to the perennial problem of dirty power. Turn off all dimmers and use only incandescent bulbs. Heating boilers, appliance motors, phone charging adapters etc all add noise. Note that your hearing is more sensitive late at night so music sounds better then.