Contrary to all belief this depends on the quality of the MP3 file you are listening to and the volume it was recorded at and how much effect was recorded on the track.(Reverb etc etc.)
I went thru this and then started remastering songs to play on the bike.
I raised all my track levels to 96-97Db (Did a lot of other things to numerous to mention)
So far 1175 CD's re-edited to play on the bike.
You will also notice that some songs sound better when the ambience is turned on and some better with the ambience off. This is a direct result of 2 things
Like I said how the song was recorded and what attributes your player has.
Some players have variable settings for effect like.
Rock, Jazz, Trance, Pop etc. Each of this provide a different ambience off the player into the bike sound system.
I have 3 players they all handle the music a little different. I stay away from Apple (personal preference, it's an Itunes issue) but they have a quality product.
A lot of people blame the speakers the system etc etc. The biggest problem is the quality of the track you are putting into the system.
The 2001-05 Wings run a 35W system with 3 Ohm speakers. The 2006 run an 85W system, not sure of the Ohm speaker output.
So on 06's the music input is a lot less critical then previous.
After I did all of this stuff( I need serious medication) my average dialed in level at 70MPH runs 19-21 on the bike volume dial and I run OEM speakers in the front and big bike in the rear (The 2 additional speakers make a big difference)
Next year I will change the speakers in the front to J&M 100W @3 Ohm. I 've had 3 different sets of speakers on the front of the bike. I went back to the OEM (Long Story)