Attenuator on a subwoofer

[VeeArSix]

I recently got a Hsu VTF2-MK4 after my Epik Sentinel crapped out. I use my home theater for both music and movies. I use the front 2 speakers and sub for 2.1 channel listening through an Emotiva USP-1 pre-amp and use the home theater bypass feature on it to also use the front speakers for movies.

I set the gain on the sub to about 12 o'clock to get the bass level to my liking for listening to music. The problem is that when I did an Audyssey calibration (onkyo 707), It gave me a -15db trim level for the sub. I don't want to have to adjust the gain on the sub each time I switch from music listening to movies, so I looked into possible solutions and read up on attenuators to reduce the subwoofer signal level coming from the AVR.

1. Will a line level attenuator work on the sub output?
2. If so, will it in any way damage the sub?
3. If this is safe to use on the sub. I looked up a few on Amazon (links below). Any body have any experience w/ these?

http://www.amazon.com/Harrison-Labs-.../dp/B0006N41B0