I can't speak to what difference it makes - I have the LFM-1 (same sub but without the port plug and extension/output switch). I'd be curious to know what you think, though. How deep it will go depends on your room. The human ear can't actually hear below 20Hz, but if it maintains output that low you'll still feel the bass.
The SMS-1 provide equalization for the subwoofer only. It's of greatest benefit in spaces that have acoustical problems (either dips or peaks in frequency response, although like any EQ it's best with peaks) that can't be resolved by adjusting sub placement or adding acoustical treatment. If you're considering an SMS-1, I'd recommend reading through the
Outlaw Guide .