There is no such thing as a 14V amp... Car audio amps are speced for different settings in the past. Now-a-days they are speced at 14.4V which is the standard for your vehicle. Stop thinking there's something different with your amp. What you're probably seeing is the ratings at the different voltages. At 12V the amp will be able to produce x power, at 14.4V it will make a little more. That's just the fact of how they work but there is no way you 'wire it up' differently. That basically means 12V (running only off the battery) and 14.4V (running when the engine is running, off the alternator). 13.6 is normal for most charging systems