I noticed someone mention FM_Boost settings in the players config.ini file

Not sure if this is the setting you're referring to when you say FM_Boost, but fail to find anything on FM_Boost when searching the BBS, so...

volume_boost_FM=nnn in HiJack sets a volume adjustment to be made when switching to the tuner (FM mode). There are also settings for AM, AUX IN and MP3.
See this entry (towards the end) in the HiJack FAQ.

/Michael