There have been some good suggestions here.
I saw some mention of trying DR Strings.
If you are going to try DRs you should look at Pure Blues they are the best of both Nickle and Steel.
If you are already concerned about brightness I would avoid Low Riders and HiBeams will probably have a "break-in" period before you like them.
IDK what style you are going for or if you are married to round wound strings but Flats and Tapewounds can be a nice way to "knock the edge off".
If you are just looking for balance you might try a different gauge. Most Boomer varieties are 45-105 you could try sets with heavier G's. I know D'Adderio makes a set of 50-105s and GHS Heavy's are 50-115. Going to a heavy set will make all the strings bigger and may not fix your balance issue, plus changing gauges will require an intonation adjustment and on the bigger strings, may require a nut slot adjustment.
You might also try a treble side pickup adjustment, you may be able to get the strings balanced by lowering the pickups on just the G string side and/or raising on the E string side.
You could also consider a bass modification. Your tone knob "rolls off" treble frequencies based on the size of the capacitor it's connected to. Moving up to a .033 or .047uf capacitor will help unbrighten your tone but will require some soldering. Stellartone make a multi-select tone pot called the tonestyler which would give you several uf options but is still a mod that requires soldering.
Lastly, you said in the OP that you tried EQ and it makes your tone muddy. That might be because you're not taking the best approach to EQing. Here's a
link to a post with some EQ guidelines.
However, I found these Rumble amp specs:
Between Gain and Drive are three preset button-activated EQ shapes: Bright (adds ‘presence and shimmer’ at the top end via +13dB@10kHz), Contour (scooped mids, or -13dB@670Hz, +1dB@80Hz, +2dB@8kHz) and Vintage (which squashes your tone and darkens the midrange via up to -9dB dynamic compression with low pass). There’s also a simple four-band EQ numbering Bass (+/-15dB@80Hz), Low-Mid (+/-12dB@280Hz), High-Mid (+/
[email protected]) and Treble (+/-15dB@10kHz).
You'll notice there is no adjustment for 2k-4khz which is probably where your issue is, so the addition of an EQ pedal or multifx unit with (semi)parametric EQ might help you dial-in a better tone.
Sorry, I know that is alot.
