problem started 2 days ago no changes have been made all.All excactly set at what it was 3months ago(all controls. Sub is a dd 9518 powerd by us amps 2000x's wired at a 1.33 reactive lowed( rougly 2500 to 2800watts of uncliiped power.).Boxed is a ported desighn 6.5cf tuned to est 41hz. Problem is sounds like sub is bottoming out but after further inspection and measurments all at dd specs. No issue's with sub mouthing or bacing all checked. Unbolted sub checked coils and all are in later stages of breaking in but just what u would expect from great american copper(brand new sub been in use less then 3 months not my first subs ...was properly broken in with noproblems or specs becoming unstable or unsutable)no coil slap or visible coil issues at all.Spider is super siff with no cone or surround damage. Motor structure is in tack and is in line and within dd specs of expected tolerance. im kinda lost usnsure of what may have happend. no changes other than batt upgrades to a 16v ( let me state elecrtic system was already supporting a 16volt syem just a simple batt change to a higher ccc rating... Any and all help would be a appreciated.. Any need more infoneed just ask. aslk alled frinds ( some compete and know there stuff and we found no issues and if they cant figure it out then no need to take to a hack *** local audio shope that has a 10 yo trying to sell me some world holding kicker l7) waste of time so hopefull some one can help me.. batt changes we done 2 months after new problem ocured... p.s no voltage change no clipping at all electric stem stays above 13.1 at full tilt. All info needed..P.S port is dd specifc. custom measuments were followed froma trueexpert's advice.

 

could the woofer be slapping the speaker wire in the box? Does the sub make any strange noises out of the box?

 

Are the tinsel leads woven in the spider?

If I remember correctly DD uses basically 10 gauge wire to hook up to the coil, but I have seen some tinsel lead problems do some strange things.

It sounds like you are exceeding the mechanical specs more then anything, but it looks like you have done everything right. Have you check with DD to see what the mechancial Xmax would be with that setup?