did i kill the bass?

I have a 98 grand with the stock infinity stereo. I have replaced the door speakers already and put in some subs in the back. I used some gator clips to for the input for the suds through a line level converter. But in the last 8 months I burned out the left side of 3 converters and finally had a sub crap out. well got new sub-new converter but all sound fine when it less than half way up but now when I turn it up more than that the bass all but disappears and sounds distorted. Even with the gains up all the way ( just testing!) Still hardly any bas. But  lots of bass at less than half way up. And it sounds the same when I disconnect the subs entirely. I used to pound Really good with just a little bass on the EQ but now I hate to turn it up even to half.Would a dead sub burn out the converter like that and crap out the radio's bass? It's like the + and - are crossed when I turn it up but I have double check everything and there are NO wires crossed. And Ideas ya'll other than getting a new head unit?