The bloatedness or boom is most likely down to a combination of sub position and seating position. You see, more than any other speaker type, a subwoofer puts energy in to a room that is often very peaky. By that, what I mean is that if you were able to play a single tone and then walk around the room, you would notice that the volume appears to rise and fall in relation to where you are standing. Changing the frequency results in a different distribution of peaks (boominess) and nulls (places where the volume is greatly diminished or even where the bass note sounds almost silent). What you are hearing is the standing waves in the room.
Standing waves are always worst at any even fraction of a room dimension. Your room is 5.5m x 5m x the ceiling height. We'll deal with just length first. The worst places to sit in your room will be half way along its length, and 1/4 way, or with the sofa up against the back wall. The same goes for the width, so don't sit at 1/2 the width, or 1/4, or up against the side walls. More even bass will be heard when you're sitting at 1/3rd or 2/3rds the room length, if that isn't practical then try 1/5th, 2/5ths, 3/5ths, 4/5ths. The same principles apply to width and height.
By this point you're probably building a mental picture of the bad places to sit in a room, and you may have already worked out that the room corners are the worst of all. It follows then that the bad places for you to sit as a listener are also the bad places to put a subwoofer. The room corners produce the most peaky in-room bass response of all. Moving the sub away from the room corner to a 1/3rd or 1/5th wall length position will smooth out some of the peaks and troughs in the bass response. Do that, and tweak the seating positions too, and you may find that your sub sounds a whole lot better.
Now, the timing.
Once you have a smoother bass response, then it's time to turn your attention to the Delay (A.K.A. Phase) feature. You may have used the AV receiver's auto set-up wizard. They're clever, but not infallible. Play some music with a well defined bass beat. Start with the phase at 0 degrees. Adjust the phase control and listen for when the bass is the strongest. This is when the sub is in-phase with the main speakers. Depending on your receiver or pre-amp, you may be able to make this adjustment from the amp rather than the back control on the sub.
In making these small adjustments to sub position, seating and then re-adjusting phase to suit the new sub position, you'll do more to tighten up the sub's integration with the main speakers and smooth its in-room response than can be achieved by simply swapping the sub for something more modern and throwing a lot of tech at the problem. If we were going the whole hog, then the next stage would be bass traps which are physical room treatments to help deal with the standing wave energy and further smooth out the in-room response. However, unless you have a dedicated room and an extra chunk of cash plus a very understanding significant other then doing a full room treatment isn't a step that most people are willing to undertake just on the say so of someone on the 'net. That leaves the neater but less effective option of electronic toys.
@hornetstinger mentioned subsonic filters. They can be added in-line on the signal feed from the AV receiver to the sub. I'll let him explain more about what they do and the benefits and costs.
The other thing to have a look at is a product such as
Anti-mode. This is a device that measures what the sub is doing in the room and applies some very accurate room correction processing. Most AV processors with room correction software built-in can do a reasonable job with the main speakers, but they either don't work on the sub channel or are very limited in what they do. Anti-mode is a dedicated to the sub channel.
There are limits to what any electronic correction system can do though. They can tame the excessive peaks reasonably effectively, but where they're limited is in dealing with the troughs where the room is sucking out the bass at specific frequencies for certain listening positions. Increasing the drive at those frequencies will never overcome what the room and physics does to sound waves. One solution is to use multiple subs.
Two-, three- or four subs in a room can help fill the troughs if the person setting them up knows what they're doing.
