Ya, what he said... and then some!
Mind, I'm not an expert here, but I'll share what I can.
Room treatments are the last thing I would recommend doing in pursuit of good sound. And most will not help with your issue. Moreover, they are useless without proper measurement equipment and acoustic knowledge.
LF soundwaves are big long and unruly, and our rooms do nothing to help out. What you are describing is, I think, a prime example of standing waves. (If I am wrong, somebody please correct me!) This is where the soundwaves as they bounce around your room are meeting up and canceling at certain frequencies, and which can occur at specific places around any given room.
To combat this, the first thing Wiliam suggested, playing with your crossover, may help a little. I think you mains are rated at 40Hz: I would do 80 for them... 1 octave up. You can try 60, but I don't think they handle lows very well. 90-100Hz may work, too (if 90 is an option). As I tweaked mine, I settled on 80 for myself as I heard both better bass and highs. It was subtle, but an added clarity existed in the SQ.
More likely, though, is location. Times 2! First, your listening position. I can safely say you don't want to be in the middle of the room, front to back. Unless of course, it just works. (Which it can.) I know a lot of folk want you to sit in an equilateral triangle with you speakers. I think you should sit a little further back than the distance between each speaker: say 7' between, you could try 8-9' away from them. Again, that's just me. But also playing with you position in the location of your room. if your LP is in a null. Move it!
The only way you can change a frequency related standing wave is by moving the location of the sources within the room. If you pull your speakers forward 1', and spread them out a little, you may effect such a change. I get space there is tight... just using that as an example. but that does bring me to Location Pt 2:
MOVE YOUR SUB!
I personally think that front loading and corner loading does not result, necessarily, in the best sound. The best location for good sound is room dependent. If every room is different... then you can't just throw Baby in a corner and call it good.
How do you find a good location for the sub?
The Subwoofer Crawl. This technique is a little crude, to be fair, but it will do much to teach you about the way bass sounds and behaves in your room. You've already described uneven bass response. With one sub, you will always have that. But you can make it sound great at you LP by doing the crawl! While you are doing it... look for 2 or 3 places around your space where you get good bass response, and then when you add a second sub, you will know where to put it.
And that is how you start to even out bass in a room. You need multiple LF sources. The goal being to excite as many room modes as possible, which results in minimizing standing waves, bass nulls, etc. I am a fan of the Geddes approach to multiple subs: that you account for all LF sources, including full-range tower speakers, and you strategically and asymmetrically place subs around the room in order to achieve that goal.
The ultimate step would be measurements, but that will take time, equipment and skill. However, using your ears is free.
Now, William mentioned one more thing: your sub. I know that I've heard good things about the new ML Dynamo subs, though I have no experience with them. I'm not gonna tell you to guy buy a new sub, though it would help. If you choose to explore multiple subs, you want to have matching subs.
There's a lot here... but acoustics can be a huge can of worms. Experiment with placemnet of everything from your LP and speakers, to your sub! That is all free, and just takes time and patience. Most people can get a room dialed in reasonably well doing that. The next step would be a Umik and REW.
Hope this helps get you started!
