I've been experiencing trouble with my sub the last few months.  It will intermittently stop working, then i'll have to unplug the my RCA from the Left/LFE jack and plug it back in.  It will then work for a while and stop.  I have the sub set to be always on (not auto detect).  It is only 6 months old.  I am using a Yamaha HTR-5940 http://www.yamaha.com/yec/products/productdetail.html?CNTID=200504 as my reciever.  My RCA sub cable is an AR Acoustics, so I am not sure if I sould try a better brand or not.  I am by no means an audiophile and any help or suggestions would be appreciated. 

You can always try any RCA interconnect cable you have to see if the cable that you have hooked up currently is faulty.  That would be the first thing I'd check.