Kenz, it sounds like they wired the amp to be on at all times. There's 3 power terminals on the amp B+/REM/GND, B+ is straight to the battery (w/ an inline fuse), REM is the "sense" wire running to the HU to determine when to let the amp's internal relay to flip on, this is the most likely issue, GND should be connected to the chassis somewhere very close to the amp (I say should because some people don't). Knowing that the HU still works but doesn't change the amp power consumption means there's main 2 possibilities at this point:
The internal circuitry inside the amp is either defective or they shorted it by the following possibility,
The REMOTE wire has been connected to the main B+ source wire or another B+ from another harness, this is the wrong way to install an amp wiring kit as it leaves the amp ON at all times. It's also a sure fire way to void your warranty and damage the amps circuitry inside as that fat gauge wire for B+ can support over 100amps of current vs the .5amp MAX the REMOTE terminal was designed to handle, thus shorting internally and never allowing the amp to turn off.
You can check the function of this "sense" wire by disconnecting it from the amp and turning on the radio and playing something, if it worked, your sub(s) won't play with the music. MAKE SURE YOU TAPE OFF THE LEAD AFTER DISCONNECTING IT, as it sounds like they wired the "amp turn on" lead to the main power wire (the fatty) which would cause your amp to pull power at all times. If this is the case, take pics where you can, bring them to the store manager and if they don't provide a FULL REFUND in cash or to your card (not a BB gift card), call corporate and explain how they voided the warranty of your product, required you to spend more money to have Subaru verify they didn't fry the radio, and could have burned the car to the ground by half assing their job not to mention purchasing new batteries due to the faulty installation practices they provided.
Sorry to hear you got to deal with these guys like this, let us know what happens. BTW I'm not kidding about the burning the car to the ground either, BB has done it before, and shorting those leads has been known to cause fires before. Technology has helped prevent these fires in newer models but it's not an excuse regardless.