I just got an alpine CDA-7876 from my brother for about $100. At the moment I use my stock head unit to power my subwoofers(2 treo12SSI). My amp's gain is about 1/2, and it gets warm pretty quick. It is an US200 by USAmps. Now my question is that in the thread title...will an aftermarket head unit give more output to my amp than just the signal I currently get from my rear speaker? Thanks Guys!
From my own experience, When my car was completly stock the sound was mediocore at best. I replaced my headunit first, the sound was alot cleaner and had much more responce. I upgraded my door speakers next, threw some Infinity kappa perfects in back and some JBL GTO 6.5;s up front, first tim ei listened to it i was amazed by the differece a set of speakers and headunit could clean up the sound.
So to answer your question, it should make a big difference...
Right now my alpine cda-9830 is all I have in. I wouldn't say it produces more bass for my stock speakers, but the sound is EXTREMELY clean. I noticed it instantly, as did my brother. I can't wait to put my speakers and subs in. I don't know how it will affect how loud your subs are, but they will sound better, no doubt about it.
What the new head unit will do: 1)Using the preamp output, you will get better sound quality for the amp, and it will drive the amp better and provide lower distortion and less clipping 2)The head unit will provide more control over the bass, meaning you can use the EQ to get a better sound curve, and also the head unit will have extended frequency response over the stock head unit, meaning it will likely go lower and be cleaner doing it.
What the head unit will not do: 1) Make your system instantly louder. If your amp is getting warm, it's either because the gain is set too high, or your stock head unit is clipping the signal, causing the amp to clip. Remember that the gain is not a volume control, it is to match the level of the amp with the head unit's output. Usually the gain will be set between 1/4-1/2 with decent head units, but since you're driving the amp with a high level input (rear speakers) it usually needs to be even lower because the high level inputs are a stronger signal. If your gain was set correctly with the stock unit, it will be the exact same volume if you were to set the gain correctly with the Alpine unit. The main change you'd get is sound quality, clarity, and extended frequency response. If you want louder, you'll need to add more power, different subs, or possibly put your subs in a ported enclosure to get some more output out of them.