My firned got a 12" subwoofer from a professional DIY guy. It has 12" Peerles xl and Hypex DS2.0 amp and is a 40x40cm cube. Goes down to 18hz -6db. Problem is that when playing low bass songs the subwoofer seems to have somekind of razor mechanical sound. Over 30hz bass sounds good, but under it sounds like you can hear the cone moving and some buzzing.It sounds colored and not clean at all. You can hear the same odd mechanical sound coming from the main speakers also. Crossover is set to 100hz. Does it matter what is the source you are playing from? We tried iphone 7 with bluetooth. Tv own youtube. Ps4 spotify and from imac online tone generator. What could be causing this? Sub is connect via a single subwoofer rca to the amp. Also tried different positions for the sub also. I have noticed that my own bass sounds arent as clean when playing from tv compared to computer.