Audio Systems: Alpine head/ kenwood amp prob.., alpine cda 7894, channel amp

Question
QUESTION: I had a 5 year old Alpine CDA-7894 that just to replaced by a new CDA-9885 due to the old head unit not generating any sound from the subs until  you turned it up to 16 or better then the subs would work. Well, the same thing happened with the new 9885.  Yes, there is continuous power to the amp..the power light never goes out.  My guess was that the problem was at the head unit and not the amp.  Amp is a 4 channel only driving 2 12" nexus subs.  Could this be in the amp, grounding issue??  I have the LPF on the amp set at about 80hz per channel, but there should be bass no matter what volume setting, but under 16 to 13 then cuts out completely.
ANSWER: there are a few things that could be the problem here. the simplest is that the gain knobs on the amp are too low.  the amp should have little holes with adjustments in them or little knobs.  they should be labeled as gain.  set them all to the middle to start and see if that helps.

if it's still too low put on some music and look at the subs. see if they are moving but not really making any bass.  if this is happening then the subs are wired out of phase. they are pulling and pushing at different times so they are canceling eachother out.  we can fix that easy by switching the + and - of one of the subs so try that if you notice they are moving but not making much sound.

Id start there.  let me know how it goes please.


---------- FOLLOW-UP ----------

QUESTION: First off Thank you for your reply!  This is a Kenwood 4 channel amp KC-746??  I think, and I had it bridged.  I switched it to stereo "a" channel and seems to work continuously so far.  Kinda stinks as I am used to a bit more punch from the subs being bridged.  As the old saying goes something is better than nothing at all!  Any idea why it would periodically come in and out?  I messed with the gain while bridged and still nothing.  I check the polarity of the connections and they were all correct.  that is the latest....thanks again!

Answer
well.  a 4 channel amp should NEVER be used to run subs.  that's part of your problem.  do you want me to recommend an amp for you?   power acoustik has great little amps for cheap that will pound pretty good!  I just installed a single 12 into my friends jeep with a power acoustik OV2-520 (amp was $80 new!) and his rear view mirror vibrates so much you cant see in it!
If so I'll need the exact model number of the subs (EXACT!) and a rough budget for the amp.