Here's the big "problem" with an untreated room, any room really.
Any frequencies that create a "null" at the mix position
will always be "problematic".
Yes you can listen to the "best" mixes ever, and "learn" the room.
BUT, if say 250Hz - 350Hz nulls,
your mix will never get it right; because you can't "hear" it.
If it's "weak", it nulls; if it's too "strong", it nulls.
Run a frequency "sweep" test, and find the "nulls"; focus on treating them.
Dave (bitflipper) "opened" my eyes to this when the "locals" used to meet for a "Sonar Users Group".
Dave had a cwp file that was a "sweep" test; mic at the mix position and record the sweep; analyze.
I think the original wave file came from the bass traps guy Winer; is that right?
Do I agree;
listen, listen, listen (kind of like location, location, location in Real Estate), hell yes!
But a "null" will always remain a "null", 0 dB SPL or 120 dB SPL; you won't "hear" it.
Just my nickel 98 here.
T