This looks very interesting. I have a 2011 E92 HK/L7 system and have been thinking about my various DIY options for improving the sound. I have the following equipment lying around:
PPI PCX-480 4 Channel Amp
JL Audio G1300 Monoblock Amp
JL Audio 10W1V2-4 (in carpeted trapezoidal enclosure)
Dynaudio 8" Mid/Woofer Pair (Left the tweeters in the old car because they were glued in)
My current idea is to use your harness to connect the G1300 to power the stock underseat subs, as a pilot project. The amp seems small enough to conceal completely, but I have not removed the trunk lining to see what space is available. Does this sound reasonable? Any other recommendation?