Often in setups with subs and or midbasses that have overlapping Pass bands you end up having to juggle gain, phase , and equalization settings in an effort to obtain the best results. The sweet spot on the volume control that you describe makes perfect sense. When you tackle an acoustical arrangement like this you have to keep in mind that your brain processes the information that it receives (sounds that you hear) using a short list of prioritized objectives. This prioritized list is as follows.
1. Frequency response of fundamental sounds. This is the first and most important. You cannot recognize any sound with out some reference to frequency (i.e. a bell dose not sound like a bell without high frequency reproduction)
2. Frequency response of harmonics of fundamental sounds. This is why your subs in the back make the music sound bigger than life. After you have successfully reproduced the initial sound if you also reproduce the lower harmonic on top of that sound you validate that sound for your brain and make it believable. This is why music over a cell phone dose not sound right.
3. Phase coherency. The phase angle will change with frequency per speaker in your system. This is ok as long as it happens gradually and it dose not conflict with the phase response from another speaker. An incorrect phase angle or a sharp shift in phase angle with in the response of a single speaker can cause your brain to red flag the signal as being phony.
4. Group delay. You brain constantly times the incoming signals in an effort to sort one sound from another. When there is a time delay that occurs within a single sound your brain rejects it, because your brain only accepts sounds arriving at the same time (actually with in a small window of time and depending on frequency) as valid.
My guess is that if you are hearing the effects of group delay from your rear subs that the box size is large. A box size that is beyond the sweet spot of a particular sub will cause poor response times. You might want to consider reconfiguring your box taking predicted group delay into account. This can remedy your timing problems entirely. I suggest a higher order enclosure that will give you the low response and good group delay combination that you are looking for. This might be a vented or electronically assisted desighn. The easiest and most obvious approach to your situation though is to use your crossovers to eliminate as much frequency overlap as possible. High pass your front sub (bandpass). You will likely have the best results with higher order asymmetrical crossovers. With an asymmetrical crossover you can create a gap between the high pass frequency of your front sub and the low pass frequency of your rear subs. In a typical car audio system this can be advantageous do to the cabin gain of your car heavily modifying your speakers actual frequency response in spite of your crossover settings.
You will always have a sweet spot with the gain on either sub. Aligning the crossover frequencies and the phase though will increase the size of that sweet spot letting you get the type of sound that you want at will.
The way to obtain smooth phase response and acceptable group delay is to model your subs in a good box program, and weigh your options.
The way to obtain the best crossover frequency settings is to RTA each speaker independently. This way you can see the effects of the crossover in play as well as the cabin gains of your ride. You will be surprised at the results.
Time delay is also a way of approaching this problem. Though you should be aware that this strong tool in its self but will not likely get you what you are looking for. If I was in your situation I would absolutely use time delay after I optimized everything ells.
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