[SteveCallas]

From what I've read, I think I can go with one 18" SW, or two seperate 15" SW's. (either option being LLT's)

Two IXL 18s as Mike mentioned would be really nice - there's also a 21" Maelstrom that will be coming out in a few months - not sure on your time frame. You also have the Maelstrom 18", the CSS 15", the FiCar Q18s, the Shiva X 15"......lots of options.

Would it be possible/advantageous to go with two seperate 15" SW's, with two different tuning points, one @~15htz and the other ~12htz. point being two different low freq tunes?????????

Definitely possible, but not advantageous. If the bottom ends are mismatched between two subs, then the summed bottom end output will be weaker than higher bass frequencies.

but in the "LLT explained" thread, Steve states that I would need twice that volume for an 18".

It varies from driver to driver. 'Most' high excursion 18s like something at or above 600 liters, but that doesn't mean they all have to.

Oh yea, does port tune Fr(dia and length) correlate to just box volume or box volume and deminsions?

Just volume. That said, you'll want to ensure ample clearance room around both port openings.

 

[SteveCallas]

The FiCar Q18 is one of those 18s that likes ~650 liters, an 8" diameter port, and tuning around 13-15hz.

 

[SteveCallas]

If the long term plan is just a single sub, I'd keep the tuning around where Mike suggested, about 15hz. If you ever plan to build a second, I'd tune a little lower, about 12-13hz. Not a huge difference either way.

You should be safe with either tuning in regards to low frequency effects. Just don't run continuous sine waves through your sub below tuning at spirited volumes. Really, I don't know why people run continuous sine waves at all - RoomEQ does a sweep, and that yields a much more reliable FR measurement.