Hello, I have a general, novice audio question. I'd like to hook up a subwoofer to a Ferguson Hill amp+speaker system that'll be connected to my MBP. I was told by a Ferguson representative that as long as it's a powered sub, I can choose any sub. That I don't need to worry about matching impedance. Is that true? Also, do I need to match the RMS either? And, where should I set the numerous volume settings available. I will have my MBP hooked up to an external DAC (HRT's Music Streamer II), which will connect to the Ferguson amp. So there will be the volume setting in iTunes, OSX, and the Ferguson amp to consider. Where should I set all 3 volume controls--and why? Thank you for any help.