On the levels issue, I mix at comfortably low levels but frequently turn the volume down until I can barely hear the song. Then, if I can still hear each instrument, I know my mix will work.
Yep...
That's what I do with my Tascam S1010m's which are tiny near fields about 3' apart and I'm 3' from them.
I don't do this mix approach on the KRK's...those are for higher volume and pushing more air.

If tha bass line can barely be heard with with some listening concentration I run with that as my first mix.
Back to it....