On using the setup for 70% movies left, right and centre speakers are required to enjoy to some extent. Adding a sub to this setup depends on the FS, BS or sat speakers and their ability to stage the sound. If you are looking for a adhoc setup and planning to upgrade later then BS XS 2.1 is an option you may audtion and consider on your liking.
Also Sonodyne BS may pair well with Marantz given the speaker's bright sound signature, I feel your AVR will not be able to bring the best out of Sonus 2605 as specs wise it appears to be under powered. Happy audtioning and all the best!