Long time audio nut checking in..
The difference notable with bluetooth streaming vs having the data cable plugged in is the sub frequencies around ~60htz, bluetooth strips a majority of the low end from the stream, test this with some RnB tracks.

Mp3 via data connection is good so long as your bit rate is above 192~, however some loss is still notable on some tracks, namely in the crispness of the mid range.

FLAC vs WAV, have tried both endlessly, both are as good as the original, id go with FLAC for the space saving

Have loaded mine up with hundreds of flacs (64gb sandisk) sounds great.