It levels out, from what I am told, because the amp is programed to do this to protect the speakers. Unfortunately you cant do a quick fix with this. The levels are fixed in the amp and the 8 inch sub really doesnt put out anything but midbass and some lower end...
What you can do is hook up an aftermarket amp and a 10" sub and add this to the system if you are happy with the sound of the rest of it. If you wanted to redo the whole system, this takes some time and unfortunately some money.
I haven't tried to switch the 8" factory sub out and run from the factory amp because I feel the amp probably only puts out something like 50 watts in that channel. The Front and rear component speakers are getting less than 25 watts also, even though it says it is a 300 watt system or something.
I havent messed with audio for around 10 years before this and had to do loads of research for this install, but just adding the amp and sub shouldn't be too hard. You would need a line out converter, and you could probably get away with a cheap one for just the sub, the sub and the amp. Running the power wire is simple. You would get the speaker wire on the WAY OUT of the amp channel that is the sub channel and hook it to the LOC. Those colors are listed somewhere in other posts, but I can get that for you if you need. From that, the RCA to the amp and then to the speakers.