Which is the best MOH value?

i’ve asterisk 1.6.1 and FreePbx
I hear the voice well.
But the music MOH is lower than the voice and sometimes the music distorts (because i’ve amplify the volume). How can have a good quality of volume?

Now i use:
MP3 format

I’ve used classical music (is suitable for this type of service).

I’ve used Adobe Audition and have amplify the music to max 0db (over distorts)
But there are many points where the music have -14db. Which is the min db standard that can i hear in a phone?
I must serch music without high and low sounds where all the volume is equal?
MP3 is the best format or must use PCM as the voice?

Thank you.