A9X started following 2x 18" LMS Ultra 5400 in 27 cubic foot 15hz vented enclosure
Quest QA4004. Australian made class G 2400W/ch/4R and about 38kg. Tough as hell and often available locally for not too bad money. I haven't measured it but I have the full schematic and there is only one series cap with a calculated F3 of 4Hz. That's good enough for me. I have 2 units and soon each channel will be powering a sealed FTW21 in about 300L ea. The QA3004 is also a great unit and is 1400W/ch/4R with similar great build quality and about 10kg less weight.