Think you could possibly be right on the overkill (if there is such a thing
), it's hard to give the correct dimensions as like I say its in a roof space but it has a 5 metre front to back by 8 metre wide floor. However the vertical space is in the triangular shape of the roof with it's highest point being aprox 2.7m so most of the 8 metre wide floor space is tucked away in the eves so really theres more like 5m x 4m of usable floor space that you can actually stand up in if that makes any make sense!
Ah man I haven't even had a chance to start looking into those yet, the Mrs keeps making me work on the stupid house renovations when all I want to do is plan my HT out but yes I was definately thinking beefy!
Never built a sub in my life so correct me if I'm wrong but thinking I could possibly build the cabinets with a sloped back (if I can figure out the maths!) so the back of them could fit into the wasted empty floor space in the eves. I was also thinking I could also fit some pretty thick acoustic absorbers in these wasted spaces, without going overboard on them obviously.