Room modes: absortion, bass traps, Eq, speaker position... what really works?
 

my room is very small for a 15 inch speaker that I use, the 38Hz - 350Hz is really a problem because of the room modes interaction with the speaker, anyone solved this kind of problem with room treatment or eq? I would like to hear some...

Have you measured it? Behringer mic + phantom power amp / Holmimpulse SW will do.

I use two Anti mode DSP units on my Tapped horns. I had 15dB peaks and troughs before at volume and DSP certainly fixed them up to acceptable +/-5 dB in room at listening position.
DSPeaker-Products
No connection with them other than satisfied customer.
The Anti mode I have is only good to 150Hz I think so meant for subs really.
SQ is good.
I believe they do a more comprehensive freq covering unit now. But that is a leap in cost and DSP the whole freq range.

I think proper bass traps would help too.
Something like the diagrams to be really effective.
RAM400-1000 Corner Bass TrapSound Acoustics

Depends how far you want to/can go.

You need to do all of it:
1. Start with speaker and listener position to get the best overall response
2. Add absortion and bass traps. You may be surprised to find that bass traps do not change the response all that much...
3. Fine-tune with EQ

yep, it's a whole package.

Have you calculated the room modes already? I should start there.

first you need to measure your room, acousticly or dimension.
Find out if you have modes (lets call it response peaks).
If so use tuned bass traps or ressonators to damp these frequencies.
Then use as much diffusers as possible. Try as wide range as possible, back wall, sealing, side walls and front wall.
No high frequency absortion should be needed (sound will go dead without some reflections).
The end result should be incredible.

Multiple subs, nearfield subs or a Double Bass Array (in a rectangular room) will all work very well without drastic acoustic treatment.

My experience so far has been that using 3+ subwoofers is far more effective than anything else with low modal frequencies. Room treatments can help higher frequency modes if there are any left. EQ is worthless for room modes.

Multiple subs would be a more complete solution but at the very least you could use one sub (could be smaller than 15"), covering your problem frequencies and working with the mains. Don't cross the mains, leave them as they are. Use the sub location, polarity and level to find an improvement.

What?!?

That's swimming up stream but you're right.

EQ can effectively deal with peaks (and dips to some extend) caused by room modes below about 100Hz.
Here're 3 articles worth reading: Bass Integration Guide – Part 1