Take a look at the Room Mode Calculator (I converted 4x6x3 meters into feet and inches):
Your room dimensions present a difficult problem to overcome. Note that the second order standing wave associated with the room width is 87Hz, and the 3rd order standing wave associated with the room length is also 87Hz. This means that the 87Hz mode will be overwhelmingly present in the room, which could account for the bloated bass you are hearing. (Also note that 58Hz is a standing wave associated with the length and the height of the room, which is also a serious issue.)
One way to address the 87Hz issue is sub placement. On the front wall, try placing the subs at the 1/4 and 3/4 points of the room width (i.e. 3.25 feet from the left and right walls). Same goes for the two rear subs. Another option would be to place one pair of subs along the opposite side walls, either 3.25 feet from the front or back wall (i.e. the 1/6 or 5/6 points along the room length). To address the 58Hz mode, place the two subs along opposite side walls at 4.75 feet from the front or back wall (i.e. the 1/4 and 3/4 points). Ideally, place the two front subs to address the 87Hz mode, and the two side subs to address the 58Hz mode.
If you can't re-locate the subs, my fear is that you will not be able to address either mode by any other method.
Don't listen to the naysayer who feels that a room measurement system is not needed. I looked at your home theater build thread, and you obviously invested a lot of time and money to build a really outstanding listening room. Having gone this far, it would be a shame if you didn't develop the skills to assess the audio response in the room. If you don't, you will never be able to finish your room acoustically, which would be a shame.
Omnimic is a simple measurement system, and a good one. However, you might also want to consider REW. I know you were somewhat intimidated when you visited the Home Theater Shack web site. However, we here at AVS have created a thread here, which is intended to make REW simple for first-time users like yourself. You should also take a quick look at the REW Guide, referenced in a link in my signature, which is a step-by-step guide for beginners. The cost of hardware is only about $100 to get started, and you can receive guidance from the many thread participants.
Good luck, and I hope you find the solution for your issue!
Can you help me with the new room sizes I posted please, I just want to see if I can move the subs into an even better position please up the front?