You can reduce the deliterious effect of having room modes close together by adding acoustic absorption in the bass region. The good news is that with your room dimensions, the biggest problem frequencies are relatively high (140Hz & 280Hz), so bass tools such as Tube Traps will work reasonably well. By providing bass absorption, you will reduce the Q of the room which will reduce the amplitude of the peaks and dips and spread out their frequency range. The only disadvantage is that this will be you'll need more bass energy from your subwoofer to create the same level of bass in the room.
If you need absorption of a specific narrow frequency range, I'd recommend RPG Modex panels. These are relatively expensive, but very effective at low frequencies. Tube Traps work well above 100 Hz, but not particularly well in the low bass frequencies.
There are many DIY articles on the net describing how to make your own tube traps. If you're a little handy in the workshop, you can make a decent tube trap for $20-$50 (depending on whether you want fancy fabric covering).
Good luck.
If you need absorption of a specific narrow frequency range, I'd recommend RPG Modex panels. These are relatively expensive, but very effective at low frequencies. Tube Traps work well above 100 Hz, but not particularly well in the low bass frequencies.
There are many DIY articles on the net describing how to make your own tube traps. If you're a little handy in the workshop, you can make a decent tube trap for $20-$50 (depending on whether you want fancy fabric covering).
Good luck.