To me, alternator whine is the most annoying form of noise. For those of you who are lucky enough never to have been exposed to alternator whine, it sounds like a miniature siren that rises in pitch with the speed of the engine. Alternator whine is almost always caused by a ground loop. The following steps will aid you in locating and correcting a ground loop problem.
Best Buy is notorious for employing entry-level installers. If you're unhappy with the installation - they should make it right.
Ground loop isolators rarely work (in my experience) and the source of the problem should be found - instead of spending more on a product that may not solve it.
Have them check it out - and if they cannot find the cause - or solve the issue - trust in a local experienced installer.
Even though you didnt DO the install - you can follow the wires to see where it is grounded - and if all the wires go along the same side of the vehicle. This will give you an idea of where to start.
You're correct - not the same - but you can often get a sense of the difference in efficiency and passive crossover points - when (all other things are equal on the sound board) the switcher plays one set vs another.
If it is quieter - the efficiency is lower... also the highs (tweeter) are directional - so you CAN get a good representation of the tweeter performance and crossover points. The woofer - not so much.
They load from the top.
You MAY need to remove the seat - but I don't remember ever having to do it in that style Monte. Look for trim panel fasteners to remove, and you may be able to just get the rear deck trim off.
Take a look at this 5th gen Monte forum page. Someone went through all the trouble already with tons of pics.