Preamp problems

I have tried two different head units and have had the same problems with both.  The problem is that I have to turn the gain all the way up on my PPI 2360 amp (powering JL Audio sub) and still am not getting the spl expected.  I didn't even have to turn the gain half way to get a lot more spl when I was using speaker level inputs with my factory head unit.  My current head unit is an Alpine 7894.  I also tried a Sony unit with 2 volt preouts.  The 7894 has 4 volt preouts.  I tried a JL Audio 300/2 amp and had the same problems.  What would cause this?  What is the solution? 

I am not going for huge spl (I am only using one JL 12W3), I just want my system to perform the way it did when I was using the speaker level inputs from my factory head unit.  I had a lot more bass at lower listening levels than I do now.  My problem is matching the amp with my head unit, but the gain on the amp is turned all the way up.  I have never had to turn the gain all the way up on an amp before.  I called the local shop where I had my head unit installed and they were suprised.  They usually have to turn the gain down after installing an aftermarket head unit, but they couldn't think of any causes for the problem.  They just want me to schedule an appointment to test my system.  It seems like my head unit should be able to provide enough voltage.  Any other ideas?

I am using 15 ft RCA cables.  Would the signal loose strength through longer wiring?  The cables are new (Monster Cable).

hey mw,

     Obviously you aren't giving enough voltage to satisfy your PPI... this explains why your high impedence inputs provide a better signal.  Essentially they take the 20-30 watts (or however much you are feeding them) and with resistors knock down the signal to the optimal voltage level that your amplifier needs to amplify the signal.  First off, I'd try replacing the interconnect to the amplifier.  If your using a shorted set of patch cords (it happens) you could be adding plenty of resistance that you dont need.. this is a common problem.  This could also explain why 2 amps were having the same problem.  On the other hand, you just may be used to having your amp hit its peak at a lower volume level.  With as much signal as you were driving into that amp with your factory head (also considering you may of had the bass button jacked up) I could see why you think your low end is not as "loud" anymore, but it probably still is (but at higher volume levels now).  If this is your taste (And if you subwoofer level control is turned all the way up-make sure), go ahead and add a line driver, or better yet, try a phoenix gold bass cube.  A bass cube will let you have better control over your low end with constant usable frequency and electrical "Q" adjustment.  Don't worry about what that guy said about adding noise, obviously he doesn't know how to fix a ground loop or how to keep from inducing RF noise into a system, but hey, that's what us pros are here for.  Feel free to e-mail jellynutzo@aol.com