Hello churchgoer,
Based on my experience, I believe you can improve your overall system performance significantly by treating it as two systems: a bass system and a mid/treble and imaging system. Good bass response is usually the hardest to get right in most rooms so I suggest attaining that goal first and then incorporate the much easier to attain good mid/treble and imaging response after.
Up front it's important for you to know you're going to need a minimum of 1 sub in your system. In general, 2 subs are going to perform about twice as well as a single sub and 4 subs, in a distributed bass array (DBA) system, are going to perform about twice as well as 2 subs. I'm assuming you're existing speakers on stands only have bass extension down to between 40-50 Hz and you're willing to spend $500 each for at least a single good quality 10-12" sub such as a HSU, SVS, REL or another good quality sub; I'm unable to state additional good quality sub brands mainly due to a lack of complete knowledge of subs currently available. It's also very important to make sure the sub(s) you buy have variable controls for volume, crossover frequency and phase. A sub that includes an automatic room control feature with mic is acceptable but not necessary.
Procedure for attaining good in-room bass response:
1. Disconnect your main speakers and move them and their stands completely out of your room.
2. Hook up your sub and place it at your dedicated listening position. Set the volume control at halfway (12 o'clock), the crossover frequency control at 50 Hz and the phase control to in-phase ("0"). Play some music with good, solid and repetitive bass.
3 Beginning at the right front corner of your room, slowly begin walking in a counter-clockwise direction around the perimeter of your room. As you're doing this, carefully listen for the exact spot the bass sounds the best to you ( solid, detailed, dynamic and natural). Take your time, listen carefully and repeat if necessary to determine the exact spot the bass sounds best to you.
4. Once the optimum spot is determined, move the sub from your listening position to this exact spot. As I stated earlier, 2 subs will perform about twice as well as a single sub; the bass will be more effortless, more dynamic, smoother, faster and integrate better with your main speakers.
If you're satisfied with good in-room bass response at your listening position, you can continue on to the next step of attaining good midrange, treble and imaging performance at your listening position. If you'd prefer bass performance that'll sound about twice as good at your listening seat, however, then obtain another sub and follow step #5 first. You also have the option of beginning with a single sub and adding a second sub when you'd like.
It's also important for you to realize that the horizontal banding in your room of areas with poor bass response, that you noticed when moving your chair forward and backward in relation to the front and rear walls, is completely normal and predictable in rooms based on the room's specific dimensions (length, width and ceiling height) and exact locations of bass producing speakers.
The procedure I'm describing will ensure your listening seat is not positioned in one of these poor bass response bands but will do nothing to improve the bass response in these bands. The only method I'm currently aware of that will provide very good bass response perception throughout any given entire room is the use of a 4-sub distributed bass array (DBA) system, such as the Audio Kinesis Swarm or Debra 4-sub DBA complete kits that cost about $3K. It's also possible to build a custom DIY 4-sub DBA for perhaps less if desired.
This is an extremely good bass solution that works in virtually any room and with any pair of main speakers but it may not fit everyone's room décor or budget. I utilize an AK Debra 4-sub DBA system in my room with a pair of Magnepan 3.7i main speakers and consider the bass performance near state of the art.
However, utilizing 2 good quality subs are also capable of providing very good bass response at a single designated listening seat if they're both optimally positioned. Sorry for the digression, here's the remainder of my suggested procedure for utilizing 2 subs:
5. Hook up your second sub and place it at your dedicated listening position. Set the volume control at halfway (12 o'clock), the crossover frequency control at 50 Hz and the phase control to in-phase ("0"). Play some music with good, solid and repetitive bass.
6. Repeat the complete exact procedure detailed above that was used to optimally locate the first sub for the second sub. The only exception being that you begin searching, for the next exact spot the bass sounds best to you walking slowly counter-clockwise around the perimeter of your room, from the location of the first sub rather than from the right front corner of your room.
Procedure for attaining good in-room midrange, treble and imaging performance at your listening position:
1. Ensure that the distance from the floor to the middle of the tweeters on your speakers, positioned on their stands, equals the distance from the floor to the middle of your ears when you're seated at your listening position.
I want to mention at this point that I agree with Eric Squires when he stated that aligning your speaker tweeters with your ears height may only be a good starting point. I think it would be best if you contacted your speakers manufacturer and asked them for their recommended alignment technique concerning exactly which drivers should be aligned with your ear height.
Failure to adequately height match the ears and speaker drivers will compromise imaging performance and may require the adjustment of the height of the speaker stands, the designated listening seat or both.
2. Midrange and treble frequency soundwaves are very short and much more directional in relation to bass frequency soundwaves which are much longer and have an omnidirectional radiating pattern.
3. For better imaging, ensure that first soundwave reflections off of each sidewall are either absorbed or diffused with the proper placement of acoustic room treatments. Other soundwave reflections off of the front and rear walls as well as the floor and ceiling may also benefit by either diffusive or absorptive acoustic room treatments.
Finally, I should point out that I just wrote a full book's worth of suggestions for you without even knowing the exact brand and model# of your main speakers. Yes, my offered suggestions are probably best described as audio basics but you should probably at least contemplate this prior to evaluating my suggestion that you may also want to consider upgrading your couch and main speakers.
Best wishes,
Tim