Try using a program like MP3 Gain, it's a free download. Analize and filter your files by changing the track settings. It may need to be done and tested on the bike a few times. But I did it for a friend of mine and it worked great. Read the help and get familiar with the program, it really doesn't take too long and it worked!
Look on the back of the player. You will find an a/b switch on the back of the HA90. It talks about changing the setting if the volume is too low. the switch on mine is behind a rubber boot. Also I just upgraded to the hog tunes speakers in front and that helps too.
You have to open up the unit and change the switch inside from A to B, or visa versa. This will solve your problem. Check HDforums for more info.
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