Soffits angled 30 deg. in from a flat wall result in speaker baffles being perpendicular to the two legs of equilateral triangle comprised of two speaker centers and a point about 10" behind your head; and the baffle extension (the face of the soffit) should be as large as you can make it, up to about 45" in each direction from the woofer in a system with a LF cutoff of around 45 hZ - There's a formula based on LF cutoff that ensures full 2 pi response for the woofer if the baffle is at least the size obtained in the formula's solution -
http://www.quarter-wave.com/Project01/B ... ection.pdf
http://www.quarter-wave.com/General/BSC_Sizing.pdf
(don't remember which one of those has it, formula includes WB and a number in the range of 4000+ )
IF you allow your walls to form a concave surface (bending IN from the soffit face) and the soffit face is much smaller than that minimum calculated above, it can cause some "horn-loading" of the sound which (depending on the room shape/size) can cause a narrowing of the "sweet spot".
Gotta crash soon (3 am), hope that helps... Steve