lolol you are bottoming out the sub lol if it has a crossover on it adjsut it to about 10 hz high er then w/e frequency you hear the slapping at
if no avail then simply back down the bass you may have already damaged your sub by doign this though
but seeing its a tangband ive seen them get handled a lot worse so you should hopefully be fine
also when you are turning up the volume between your computer eq and speakers dont ever go full volume lol adds distortion which adds heat which creates magic smoke aka death of the speaker
and th reason why it hass jsut recently started acting up is because of a break in period some subs take longer to break in then others breaking it in is jsut making the soft parts of the sub more easily mmovable for the voice coil IE surround, spider, and tinsel leads
iff what i said had no affect then take the sub appart if you are not underwarranty press the sub down from the center around the dustcap (the outward facing semisphere) apply pressure evenly and see if the tinsel leads are touching the cone if so then you need to bend the connectors outward a bit to prevent this careful as to not bend too far so the speaker still has full play
ALSO lol you may have jsut torn the surround fo the speaker and it may be a possible fix lol i use electrical tape and daps of hotglue to fixx the tears in a speaker
but if nothing ive stated seems to be the problem then chances are youve blown it and you can jsut go out and buy a new replacement sub for it possibly somethign better though be aware if it is a bandpass box or a t line then you must get the same speaker they sent it with
if thats the case then look into tang band they were making logitechs subs awhile ago idk if they still are