The power cap should be within 2ft of the amplifier, normally. I strongly recommend dropping the power cap, as its nothing but a way of stabilizing the power drain. What you should do is get a second battery specifically for car audio. Run the power wire from the (+) POS of the main battery to the (+) POS terminal of the 2nd battery. Ground 2nd batt directly to frame. Run amp power wire from 2nd batt power terminal and ground separately from the second battery directly to the frame as well. (Dont forget FUSES! one within 6" of each (+) terminal minimum.) This increases the available power to the system and minimizes impact on your car's charging system.

The easiest and one of the most commonly overlooked problems with having subs and other high drain items is the car's stock charging system. Not upgrading that is where 99% of your power issues will come from.
Upgrade the stock wire from the alternator to the battery, and the front battery's ground needs an upgrade as well, otherwise its nothing but a bottleneck in the charging system. Use the same gauge wire that you are using for your power wire to your amplifier. I recommend having a ground both to the frame and to the engine block as well.