I currently have Q15 with the BP option, i have tried many boxes from 3.7 to 4.3 the one IMO that is best for lows is the one I'm currently running , 4.0cf @ 30hz the lows are insane. I'm currently powering mine with the sub ch. of a RF 25 to life power 1000(1000 watts @ 1ohm) The the Q with BP can take every bit of 1500 watts if you choose to send it that much... now as far as playing below tuning that's tricky. Honestly its all install dependent. I personally do not use my subsonic filter, but that's ME. I know what my install is capable of, i love hearing my sub play those low notes in the 25 to 30 Hz region. This in a Colorado ex cab, port firing to the drivers side..BTW the highest Ive hit on the term lab is 143 @ 44hz in the 4.0cf @ 30hz box. And yes there are people hitting 140's in a trunk with a Q.