I hear good things about Fi subs... Both 12" sub on each link. How would they do in a 4.2ft^3 enclosure tuned at 21hz? https://ssl.perfora.net/www.ficaraudio.com/sess/utn;jsessionid=154f7e58d0c0cbe/shopdata/0050_Speakers/0030_Q/product_overview.shopscript https://ssl.perfora.net/www.ficaraudio.com/sess/utn;jsessionid=154f7e58d0c0cbe/shopdata/0050_Speakers/0040_SSD/product_overview.shopscript