Mounting an amp to a sub box can damage the amp, it depends on a few things, if you have a cheap "unbraced" box, [lots of flex] and you have a cheap amp, [poor construction/assembly] and you crank the bass, then yes, you can do damage.However, I have often mounted amps to sub cabinets without any problems.
As mentioned, it makes stealing the amp very easy.
Mount it under the pass. seat if it will fit.
94
94LS
^^^ now in the process of being striped down and gutted for the rebuild.
Now cruising in my new winter car, 89 Chrysler Dynasty
You get what you pay for.
Nothing is impossible, the impossible takes longer and costs more money.