mtx 801d doesn't have subsonic filter. but it has compression circuit to eliminate clipping. this amp used to power subs in ported or bandpass, will it blow them (cuz not having subsonic filter)? if yes,where this filter should be placed?
you need the filter for a ported box, yes. otherwise when the amp sends frequencies to the subs below that of the tuned port frequency, the subs lose their suspension (the resistance supplied by air pressure inside the box) and they can over-extend, and that causes the subs to get fried.
www.partsexpress.com has an FMOD 20Hz high pass filter (RCA) you can connect to the amp at the RCA lines for about $25. that's all ya need