Generally, and take this with a grain of salt as I don't claim to be the definitive authority on eq, just sharing best case practices that I am aware of.
Feel free to eq below 3-400 Hz other than using caution to apply too much positive eq since we know +3 dB = double power, +6 dB = 4x, +10 dB = 10x. Above 3-400 Hz, or perhaps as high as 800ish Hz, I wouldn't apply extensive eq. Try to apply it based on anechoic measurements, and don't apply narrow filters. For example, a broad shelf filter that applies a gradual cut all the way out to 20 Khz in order to match a target curve, or a broad, mild filter to tame down a broad range from 3-6 KHz where there is perhaps some excess energy(just as an example, not in your specific case). Other than that, many advocate limiting eq above Schroeder as much as possible.
Eq boost will increase ringing, cuts will reduce it. Trying to boost a null will be detrimental.
Subs may help a little out as high as 150 Hz or so if you cross them a little higher, try experimenting as high as 120....the crossover is not a brick wall so you may affect things slightly above crossover. Above that, its probably floor and/or ceiling bounce and SBIR. I think the only way to improve that is to change speaker distance from the front wall, or move MLP. Maybe carpet on the floor with thick carpet pad. Make sure you don't have a solid surface coffee table. Things like this might be worth trying.
Recently reorganised my room and changed speaker positions, so re-did the EQ on them. I've done all the measurements but been experimenting with different approaches to smoothing in REW combined with allowing or not allowing REW to create sharp filters below 200Hz (that's a selectable tick box option). I haven't done extensive listening tests between the different EQ filters created, but I wanted to get a feel from people here as to how sharp it's wise to make filters below 200Hz, here are some Equaliser APO graphs showing some different approaches to sharpness of filters, obviously the sharper filters make it fit the curve more accurately, but is there a trade-off to be had here in terms of sound quality when using sharp filters? (so this discussion is all in relation to below 200Hz)
Option A: 1/6th Smoothing and no narrow filters:
Filter 1: ON PK Fc 50.10 Hz Gain -7.00 dB Q 4.989
Filter 2: ON PK Fc 68.20 Hz Gain -4.80 dB Q 5.000
Filter 3: ON PK Fc 74.30 Hz Gain 3.00 dB Q 4.955
Filter 4: ON PK Fc 84.30 Hz Gain -2.90 dB Q 4.997
Filter 5: ON PK Fc 157.5 Hz Gain -5.70 dB Q 4.996
Filter 6: ON PK Fc 178.5 Hz Gain 3.00 dB Q 4.804
Filter 7: ON PK Fc 220.0 Hz Gain -5.20 dB Q 4.596
Filter 8: ON PK Fc 481.0 Hz Gain -2.20 dB Q 4.999
Option B: 1/6th Smoothing and narrow filters:
Filter 1: ON PK Fc 50.30 Hz Gain -7.30 dB Q 4.757
Filter 2: ON PK Fc 66.80 Hz Gain -5.10 dB Q 13.956
Filter 3: ON PK Fc 85.30 Hz Gain -2.70 dB Q 7.582
Filter 4: ON PK Fc 155.5 Hz Gain -5.60 dB Q 7.829
Filter 5: ON PK Fc 185.5 Hz Gain 3.00 dB Q 5.438
Filter 6: ON PK Fc 217.0 Hz Gain -5.50 dB Q 3.457
Filter 7: ON PK Fc 386.0 Hz Gain 1.10 dB Q 2.524
Filter 8: ON PK Fc 471.0 Hz Gain -2.90 dB Q 4.652
Option C: Var Smoothing and no narrow filters:
Filter 1: ON PK Fc 50.60 Hz Gain -7.20 dB Q 5.000
Filter 2: ON PK Fc 66.40 Hz Gain -1.90 dB Q 5.000
Filter 3: ON PK Fc 84.10 Hz Gain -1.40 dB Q 4.999
Filter 4: ON PK Fc 155.5 Hz Gain -3.00 dB Q 5.000
Filter 5: ON PK Fc 222.0 Hz Gain -4.70 dB Q 5.000
Filter 6: ON PK Fc 474.0 Hz Gain -2.00 dB Q 5.000
Option D: Var Smoothing and narrow filters:
Filter 1: ON PK Fc 46.60 Hz Gain -3.80 dB Q 6.807
Filter 2: ON PK Fc 51.90 Hz Gain -7.80 dB Q 10.225
Filter 3: ON PK Fc 66.30 Hz Gain -9.20 dB Q 21.176
Filter 4: ON PK Fc 81.70 Hz Gain -3.70 dB Q 20.085
Filter 5: ON PK Fc 88.50 Hz Gain -4.60 dB Q 33.277
Filter 6: ON PK Fc 155.0 Hz Gain -9.10 dB Q 10.389
Filter 7: ON PK Fc 184.5 Hz Gain 3.00 dB Q 2.056
Filter 8: ON PK Fc 218.0 Hz Gain -7.30 dB Q 4.341
Filter 9: ON PK Fc 475.0 Hz Gain -2.70 dB Q 7.189
You can see that the graphs all follow the same trend, but some with more detail than others, I'm thinking the following order of preference from best to worst when taking in "Graph Detail" and associated Q-values:
Option B > Option D > Option A > Option C
What do you guys reckon, and how large a Q value is sensible to use under 200Hz (for cutting peaks)?
EDIT: did some more subjective listening tests and Option D is definitely the best with the bass being tighter and less woolly which also seems to translate indirectly into better clarity of mids, so it seems that Var Smoothing combined with sharp filters (large Q) is the best option. Seems that below 200Hz a high Q value for cutting peaks is not detrimental, and the Var Smoothing option in REW means that you see maximum detail in the graphs during the sub 200Hz part......so sharp scalpel treatment for both viewing & cutting peaks below 200Hz seems like the best combination option.