Scan-Speak 8531 enclosure size...
It is often discussed about what the optimal enclosure size is for this excellent woofer. For a vented enclosure, a number that often pops up is 22 liters with a low 30's tuning because at one time or another, an engineer from Scan Speak said this is what the woofer was designed for. Well, I've got to warn you folks: vented in a 22 liter enclosure is clearly bad advice - and it keeps getting repeated by people who think it's some sort of definitive number. What the engineers designed for and what manufacturing has come up with is often different, and in this case very different. I think I've tested about 8 or 10 8531's over many years and the average Qts is about .47 with a range of .4 to .55 - very far off from manufacturer's specs. My tests for T/S parameters are often verified with high level in-box near field response curves.

With the 8531, the best advice you'll get is to go big or go sealed. And even having said that, don't be afraid to go sealed as the 8531 has very satisfying extension in a sealed box. For vented, I think 30 liters is the smallest I would go, but 38 is even better, such as what I used in the ZRT design.