disclaimer: I spent many years as an analog circuits engineer, but i've been out the mix for a while.
i pulled up the spec sheet for RC4558P and its recommended supply voltage is +/- 5V. its max is +/- 15V, so be careful if you're using a +/- 18v alembic supply. Its current consumption rises fairly significantly at supply voltages above 5V. I would trade performance over power consumption in this case. You can definitely tell when you've put a crappy (i.e. low quality) op-amp in a pre-amp circuit, so battery usage be damned IMHO.
interesting discussion about this subject here:
https://www.diyaudio.com/forums/analog-line-level/135546-rc4558p-replacements.htmlputting a bypass cap right across the supply pins at the op amp is an old trick, pretty effective if you keep your leads short and use a good quality low ESR cap. It doesn't take much, maybe a 0.1 uF. ya gotta have a good hand with a soldering iron, because you gotta tack the cap leads right to the IC pins. I don't recommend doing this unless you're experienced soldering delicate electronic components and have the right equipment to do it, because you *will* cook the op amp if you can't get it done in under ~ 2 seconds. The intent behind this is to decouple power supply noise from the op amp and to prevent oscillation due to a high impedance power supply path. it doesn't take too much softness in the supply path to wreak havoc on the output, especially if there are rapidly changing signals with high frequency components.
i had to replace one one the op amps in my series II years ago, i don't remember what it was other than a dual op amp in an 8-pin DIP. You might poke around and see what alembic is using in their circuits, i'm pretty sure they've done their homework on these.
if you have access to a scope (i'm thinking this is a lost art these days. i work in an electrical shop and i can't think of anybody i know who knows how to drive one anymore except for me. when i draw a waveform on a piece of paper to illustrate a point, people's eyes glaze over... but i digress), it's pretty easy to trace out a hum. a good first step might be to get the op amp power situation straightened out and then go from there. if there's something funky going on there, you can spend endless hours chasing your tail trying to find it in the signal path.
good luck and happy hunting.
