very nice sub.
indeed, it has an 'effortless' authority to it, with potential for very accurate subbass that is low and loud.
for sure, the subbox design and amp are as important as the sub itself. i have run mine in a sealed box of 1.3cuft internal volume. though it did 'roll off' below about 50Hz, but i had the benefit of an EQ to flatten out the response.
and if it sounds good on your back seat, wait until it's in the boot!
my xover settings took into account that i also had time correction/alignment. this meant i could easily create a 'bass upfront' soundstage by delaying the front speakers relative to the sub (ie: allow the subbass to 'catch up' in time with the speakers). therefore, i set the LP filter relatively high; at higher frequencies, sound becomes more 'directional', and you can hear its locality more easily (presuming you don't have boot rattles). not ideal with a sub in the boot, but perfectly fine with time correction.
my LP was set at around 90Hz with a 12dB/oct slope. i initially started with a LP around 70Hz with a steeper 18~24dB/oct slope, but the IDMAX is so accurate with upper subbass, it provided some extra authority to these frequencies, and helped improve the midbass. the HP for the speakers was around 90~100Hz, also with a 12dB/oct slope. initially, it was set lower, in accordance with the lower LP filter setting. the ideal setting will depend upon the quality of the midbass from your front speakers.
so, you can easily experiment to see what sounds best.
i'm missing the sound of the IDMAX. it's sitting in the house awaiting install into my GT. but things aren't happening fast enough.
hope this helps.