[Bradley]

I am/was somewhat in the same boat as you are... arriving at the conclusion that a 650 liter sonotube wouldn't integrate well into my HT space, and instead looking at box in the neighborhood of 15 ft^3 (425 liters). I looked at a 15" driver solution but ultimately decided on the RL-P18 because of the large advantage it has in displacement, with only a modest increase in cost. The exact tuning frequency seems to be a matter of opinion and preference, assuming you stay within the handful of well established guidelines concerning port length, port air velocity, and cone excursion. At 15 cubit feet, SteveC suggested a 16.5 Hz tune with the RL-P18 D2. You can go lower as long as port length doesn't get too long and raise the specter of a first port resonance less than 190-200 Hz. I don't know that the 3000 cubit foot room will help or hurt. My understanding is that it will have a bit less room gain at low frequences than, say, a 2000 cubic foot room. But I don't know how to measure or quantify that. Either way, the 18" driver will produce ample output and presumably you can use a BFD to EQ it to a fairly flat in-room FR response curve.

Hope that helps.