Yeah, I hate to say it, but I don't have any experience with the BL's either (only the Q's and BTL's). Suffice it to say though, I think the BL's have a stiff spider configuration much like the BTL, but you may want to check me on that one. I think the stiffness of the spiders play a part in what kind power it takes to get the woofer going, but I can't be sure. I think, in the situation you described above, that it would be a better idea to perhaps go with (2) properly ported 15's. Even though the 18's have a distinct cone area advantage, I am gravely afraid that you may wind up choking the box to death if you do 18's in a large box and in turn you won't really get much out of doing 18's vs. 15's. I'm not sure that I am the most qualified person to answer your question about the height, but I'll give it my best shot. I think you are definitely on to something about the box being so tall that SPL begins to diminish because let's face it, most of the airflow will be passing across the top of the enclosure with a typical CRX style box. If your box is obnoxiously tall then process of elimination may end up dictating that the sides would be responsible for the majority of airflow resulting in perhaps a system not quite as loud as it could be if you went with say 2 15's in a properly ported box.
I don't want to make your mind up for you in any way, but I do hope what I've said helps. I hope that some of the more knowledgeable folks around here will chime in for you and provide some perspective as well. Perhaps Nick, Shawn, ng, or someone with some technical savy can help out on this one.
