The Music First (I use one) will pretty much add nothing and take nothing away. What you will hear will be the amp/ CD. It won't add warmth or whatever.
When I was selling my CJ Pv-10 I was demoing it to the buyer. I remember thinking, hey this sounds really good, why am I changing it. After he left I hooked the MF back up and I realised why. So much more good clean music and with twice as much bass !!
If you've got the front end to cope with it and don't need the gain I would strongly recommend the MF passive.