yes, Audio Tool on Android is decent above 100Hz. below 100Hz the sharp rolloff to control wind noise makes it inaccurate. you can get USB measurement mics that may work with the phone/app for improved accuracy. iphone has iAudioInterface for that along with Audio Tools by Studio Six Digital.
you can mount amps to boxes, but i like to mount the amps to a board, add a 2x2 stand off, then route wires behind the amp board. it does reduce vibration and makes the installation look much nicer. in reality, the box should be so rigid it doesn't vibrate.
here is an example of that:
2006 Hundai Elantra GT 5 door - simple Alpine system