Sorry to Sunny8888, have use his photo.... Anyway based on the photo, some cunclusion can be made:
1. Are you using all the 4 chs? If yes, maybe the selection and cabling is wrong.
2. If using only 2ch, maybe the 2ch/4ch selection is wrong, or the the RCA cable input wrong.
3. If correct then try to put the trebleand bass to minimum, switch between full and sub(maybe printing error), increase the sub gain more
4. If want to low pass it, try to use low pass filter coil, it cost around 80-100 with the value of 3.3. Maybe can get cheaper as I bought it from accessories shop. As far as I know a cap is act as high pass filter, not low. It will only filter out the bass (I use it to cut the bass to my tweeter)
5. Last resort, if still cannot, then you have to use it as front speakers amp..
Lastly, I don't own a 4 ch amp yet, maybe all my statement is wrong. So please don't flame me, but this is 1 of the way to trouble shoot, only 4 ch amp got more to trouble shoot compare to a 2ch if got this type of problem.