It's usually 120hz or 80hz and below. When your processor decodes the surround signal or takes a full range 2 channel signal it will send the bass to your subwooofer through a regular single RCA/XLR out. Some processors allow you to adjust the crossover point and slope.

Usually just the one coming from the subwoofer out of your receiver to the line in of your sub.
A select few manufacturers (such as REL) suggest using the speaker outs with speaker cable to the sub's speaker binding posts (if indeed your sub has them - not all do), but if your sub's manual doean't suggest doing it this way, don't worry about it.
You can buy a more costly dedicated brand name subwoofer cable, but it's not guaranteed you'll hear a difference over a standard RG6 with RCA's at each end.
The best way to get the uppermost performance from your sub is to spend a lot of time positioning it and tuning it in. A search on google for 'subwoofer tuning' or 'subwoofer positioning' should give you a link to the info for this.

One thing I can tell you, unless you like the bass to be overwhelming, your surround set-up do it's best work when you can only tell the sub is actually on when you turn it off (unless you have satellite speakers).

using just one channel of a R&L RCA cords?

Or do I need a specific type of cord just to hook up the LFE.

I am not sure if a special cord is needed and neither the Denon manual or subwoofer manual (Velodyne DLS-4000) say to use a single side of a RCA cord or a special LFE cord is required. I did see a cord that was L&R that went into a single cord and I am not sure if that is what I need.

your sub will interact with your room and give you peaks and valleys at certain frequencies that may not be pleasant...EQ smooths those out by targeting a response curve that is more natural (not necessarily flat) and will blend in better with the rest of the sound coming from your other speakers.

It's important to do.

for someone on a low budget...

you can pick up used BFDs on AVS or ebay IIRC.<ul><li><a href="http://www.hometheatershack.com/forums/subwoofer-equalization-calibration/">http://www.hometheatershack.com/forums/subwoofer-equalization-calibration/</a</li></ul>

As if I didn't have enough gadgets. Damn you! :)

Looks like I could use a BFD...

<img src="http://i34.photobucket.com/albums/d102/escape2music/audio/meas3.jpg">

I don't know what your preference is, but generally it's recommended to shoot for a 75dB level from the sub.

I'd goose the volume up just a bit to bring that 30Hz dip up to that level, then knock everything else down (especially that 20Hz peak) with a BFD to get yourself a nice curve down to the xover point. You'll be able to hear the sub's "wheelhouse" better: the 20-40Hz range.

It should let you play it louder without any 'boomies'. You might gain some 15-20Hz response that way too, which will help shake the room, even if slightly distorted. The BFD will let you manage the distortion by keeping the peaks in check.

That's a nice sub you've got there.

I may have just located a used one near me... will give it a try.

Some people claim the BFD 2496 is much better due to higher signal/noise ratio, but I found a good deal on a used 1124p.