The LOUD button on aftermarket head units is to compensate for bass at low volume levels. As the volume goes up, the bass circuitry reduces that LOUD circuit in response to the level...bass always drops at higher volume levels, believe it or not....in most "average" systems, bass seems to get neutral as the volume gets insanely high. This is not for competition level systems competing on the IASCA or USAC circuits -- those guys have caps, extra batteries, and the like -- but it seems on average bass reduces in response to very high volumes.

I have in my 99 Accord what is similar to what you're thinking of doing...I have two 15's in a sealed box, powered by a huge Rockford Fosgate amp, all coming off the factory radio because I like the sound of the head unit and speakers---STOCK. While sometimes I wish I had that LOUD button, I compensate for this by cranking the bass knob on the factory radio up quite a bit past halfway---this gets me low, deep bass at lower volume levels. Without a LOUD button, and if your system is going to use an RCA converter like mine does to connect an amp to the factory radio, there is no choice but to crank up the bass knob on the radio itself to get deep bass at low volumes. It will work though.