Last couple comments on this:I had 2 wires backwards on sub box, also my box don't have proper internal volume. The magnet almost touches box, I bought this box 2 years ago off eBay when I 1st got truck, its a down fire box, has 2 inch risers on each end box, I was thinking if I was going to trim the risers off and upfire box, I also have 10" sub spacers I purchased awhile back I could install onto box...
You found one sub was out of phase of with the other but didn't mention if you heard an improvement after correcting. Did you notice a change for the better when you swapped + & - on one sub?
All by itself, the "issue" of the magnets almost touching the box isn't bad. It's seriously doubtful that is the root of your dissatisfaction, it's equally doubtful extension rings will do much except cause other issues.
There is a significant difference between engineered enclosures with carefully selected sub drivers and off-the-shelf boxes that happen to fit in the space available. Having said that, based on your expectations I'm guessing there won't be a viable turn key solution to ring your bell.
You *could* find a box with a little bit more internal volume but the teeny improvement that might make isn't gonna do what you want.
You mentioned perhaps moving to a single driver enclosure, so for what it's worth, MTX offers a very nice design. You may get lucky and find someone close by with one of these so that you can hear it installed. This is the only solution I'm aware of for the RL with design engineering behind it, everything else is made under the criteria of making something that fits space available. Is this for you? I kinda doubt it. Check it out here
As a final suggestion, its very important to have confidence the polarity of connections between the head unit and LOC are correct. The reason is based in the OEM head using "floating grounds" on the - outputs. Aside from audible issues, if R & L side + & - are not correct, crossed connections can "blow" the internal amp - which may not happen immediately. An easy way to determine if these connections are correct is to check that when balancing full L to full R actually results in sound coming ONLY from the L or R sides. Do that as a final check. IF, when balancing all the way L you still hear sound from the R side, its a safe bet there is a problem between head spk outputs and the LOC.
In short, if everything in your current system is correctly connected, and you need more boom than you are currently getting from the box under the back seat, it may be time to consider removing the back seat altogether. Then you'll have room for a bunch of 15's.