Honest to god, I would rather mix on speakers that only go down to 80Hz in MY ROOM (
The Feisty Chicken Recording Studio - Birthplace of #1 Hit Singles - Silver Spring, MD) than mix with speakers that go down to 30Hz in my bedroom (no treatment). There's no question about it.
As for how flat a speaker is, it is MEANINGLESS in an untreated room. From one speaker to the next the craziest difference you could possibly expect is +/-3dB at any given frequency, max. But a typical untreated bedroom very likely has peaks and nulls in the range of +/- 15dB!!!!!!! 3dB is NOTHING in that context.
People really underestimate the importance of the room you are mixing in. 90% of what you are hearing is your room. 10% at best is the actual speakers. Just because you can't see all the f*cked up standing waves and comb-filtering phase anomalies with your eyeballs doesn't mean they aren't there. That's just burying your head in the sand. Just because you can't see that Lion doesn't mean he/she isn't going to chew you up into a billion pieces.
You can make a SIGNIFICANT improvement in any room with no damage other than a couple nail sized holes with nothing other than very simple bass traps and absorbers made from some 1x2 pine, some rigid fiberglass pieces, and whatever fabric you think looks pretty. You can hang absorbers on the wall like pictures that will not interfere with floor space. You can put bass traps in the corners that will eat up maybe a couple square feet for the whole room.
But don't mind me, I don't know what the hell I'm talking about. I just started making music last week.