The SMS-1 (and pretty much every EQ tool around these days, including Behringer's Feedback Destroyer) operates in the digital domain. In all of those cases, only the sub's signal is bounced from analog to digital and back, so it's not affecting the more sensitive full-range signals - I certainly couldn't discern any audible effect (aside from the improvements due to EQ'ing) with I installed an SMS-1 back in December. If you haven't done so already, poke around through Outlaw's SMS-1 Guide for an idea of what it'll do - not only will it provide some auto-setup, but it'll allow very convenient and effective manual tweaking through an on-screen system (no separate PC and software package required, unlike other systems).