For MP3s, you can use the program MP3 gain. It normalizes your MP3s to a set level. It comes default at 89 dB. I use 90 dB personally. It is completely reversible. I've been using that for about 10 years with MP3s.
I use the foobar tool, it applies the same tags to both flacs and mp3s, the M1 reads the FLAC's tags no problem, but seems to ignore the MP3's tags:
Example MP3
Format : MPEG Audio
File size : 9.11 MiB
Duration : 3 min 58 s
Overall bit rate mode : Constant
Overall bit rate : 320 kb/s
Album : The Screen Behind The Mirror
Album/Performer : Enigma
Track name : Gravity Of Love
Track name/Position : 03
Performer : Enigma
Genre : New Age
Recorded date : 2000
Writing library : LAME3.98r
Audio
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Mode : Joint stereo
Duration : 3 min 58 s
Bit rate mode : Constant
Bit rate : 320 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Compression mode : Lossy
Replay gain : -6.55 dB
Replay gain peak : 1.032699
Stream size : 9.10 MiB (100%)
Writing library : LAME3.98r
Encoding settings : -m j -V 4 -q 3 -lowpass 20.5
Example FLAC
Format : FLAC
Format/Info : Free Lossless Audio Codec
File size : 17.1 MiB
Duration : 3 min 21 s
Overall bit rate mode : Variable
Overall bit rate : 711 kb/s
Album : XX
Track name : Crystalised
Track name/Position : 03
Performer : The XX
Genre : Alternative
Description : Track 3
Recorded date : 2009
Cover : Yes
Cover type : Cover (front)
Cover MIME : image/jpeg
Audio
Format : FLAC
Format/Info : Free Lossless Audio Codec
Duration : 3 min 21 s
Bit rate mode : Variable
Bit rate : 710 kb/s
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 44.1 kHz
Bit depth : 16 bits
Replay gain : -6.14 dB
Replay gain peak : 1.000000
Stream size : 17.1 MiB (100%)
Writing library : libFLAC 1.2.1