cable burning

After you've stopped laughing, maybe somebody has an answer for this. I have Transparent speaker cables. At the amp end of this cable there is a black plastic cover. This is where the cable is seperated into two connecting ends....about 5" or so. I recently bought new amps and the posts are at each end of the amp. The cables didn't have enuff spread to reach. My local audio dealer said he could have the cables sent to Transparent and for a cost they would extend the ends to what ever I needed. Where I work, we make some cables for barcode reading guns. Thinking I could save a few $ I brought mine in thinking we could take those plastic boxes off and recover the cables with some sheathing and it would give me the extra length I need to reach the posts. We had a little trouble with a resister that was glued into that plastic box but we were able to make it work. I had just barely enuff length to make the connection. I have Mag 3.7 speakers and they are fairly new....about 120 hours on them now. So, I havn't been playing anything to loudly, till the other night. I decided to crank it up some. All was well until, here it comes....I see smoke comming up from behind one of the amps. A thin line like from a cigarette left burning in an ashtray. At the same time I smelled it. I hit the off button on the CD and dived to the off buttons on the amps. Upon further review I could see that the speaker cable right where we had covered that resister was melting. After waiting for it to cool off, I covered it with some electrical tape and turned it back on just to see if it would still play. No problems Sounds fine. My intention now is to do what I should've done and send them in to be fixed professionally. Seeing as how I have very little knowledge of all things electrical, anybody want to try and explain what happened?