Which songs you are listening ....? Might be they are rated at loud frequencies and you are considering them lows?
1)Also that dum sound is there because its cold now so amp capacitor got cold and when you suddenly power up it frequently charged up therefore ....
2)Moreover Its a common problem in magus ,As I listen the same sound when I used to wired my 2x 307D4 with the same amp ....Don't worry!
How your headlights going to dull when your car got horny big alternator
,Also might be your setting is not correct from amp ......Ask maddy or Splfanatic to tune your setup ! 
P.S "I might be wrong about 1st theory" 