I'll defend the 511B to the extent of saying that if properly damped it's a good-sounding horn. It's useful down to 800Hz or lower, well below what most inexpensive waveguides will happily do. Without damping my 511s did sound awful at any output over a whisper, and getting them damped took a bit of effort. Supposedly bolting the flange to a mounting surface helps, but mine are free-standing on top of mid-bass cabinets.
Being in possession of 511Bs and also being somewhat masochistic, I spent the time to figure out what it takes to damp them thoroughly:
http://www.diyaudio.com/forums/multi...ml#post3060342
For drivers I'm using Radian 475PBs, which work pretty well down to 800Hz with a 4th-order LR crossover.