I'm having the same problem. After years playing bass-friendly songs (Joss Stone, Nelly Furtado, Bill Withers), I came up in a band with two former hardcore / grind "guitards". One of them have no idea how does his guitar sound clean
We are playing a faster 70's hard rock and I'm having problems to be heard. I'm using hi-beams now, but I ordered a set of GHS precision flats to test them in this new "environment".
I have experience with flats. My first "playable" bass (a Rick copy) came with unknown flats on it. I also had a fretless Squier Jazz and a SX Precision strung with TI's. So it won't be an unknown experience.
I choose the GHS over the TI's because I'm looking for a thumpier tone. I also play stantards with a friend of mine in a classical guitar / bass setup, so they will give me a more upright-ish tone.
Well...(and of course this is IMO, IMHO and all that other disclaimer stuff

), unless someone is playing with a guitarist(s) copping that Chet Atkins clean sound, there is going to be some blending of sounds between bass and guitar. It's inevitable as roundwounds are going to give a bass a sound that is more guitar-like (which can be appropriate and even desired).
When you combine distorted guitar (as in rock music) with a bass strung with roundwounds, there is just going to be some sonic space shared. And it has nothing at all to do with volume. It has to do with tone. A distorted power chord on guitar along with a punchy roundwound bass line is going to cause some sonic blending. The sounds blend, and that isn't a bad thing, but it does explain why so many rock bass players have a problem "cutting through the mix". How can your bass be clearly heard as your bass when it sounds like the guitars? And that isn't just my opinion...just look at all the posts by bass players here wanting help doing just that.
Flats remove quite a bit of that "guitar" sound from the bass. That alone can provide some distinction between the sounds of the bass and guitar. Some don't want that and I understand that all too well, having played in rock bands for 30 years using roundwound strings. I still love that roundwound tone however.
But, I've come to the conclusion that what I want in "my" tone isn't necessarily best for my band's overall sound. Being heard among the guitars isn't all about volume, as there are numerous posts advising boosting the mids to get the bass to be heard. Flats allow me to be heard in a way I didn't consider before as I had always been a diehard roundwound player and had great disdain for flatwound strings. I still do not really care for the sound of flats when I play by myself, and honestly there are times even playing live that I still fall into that "Where is MY tone?" selfishness (on my part...I'm not saying that about anyone else). But then I realize that the band sounds better because the rhythm section sounds tighter (by that I don't mean we are playing any tighter...I mean how it 'sounds' tighter...like the drums and bass are compressed into one instrument). I know many do not like that sound (or lack of sound in their opinion). But I'm standing out more as bass player by not standing out, if that makes any sense. It's not a popular concept for modern bass players.
As a long time roundwound user, it really requires me to lose my ego for me to accept how I sound now. I've spent my entire career as a bass player trying to improve "MY" tone. Now I don't care and for some odd reason I hear myself better than I ever have. Strange...very strange.
Good luck with your experiment.
[MY LAST DISCLAIMER: I really wish everyone would understand that EVERYTHING I write is MY opinion only (even if it comes across as me thinking I'm always right) and I shouldn't have to add a disclaimer to every single point I express. It should be obvious that whatever I say can only be from my point of view. Of course that's just my opinion.

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