Steve,
I went down this route and although it has improved things for me (a lot) you may find as Ian suggests you don't need to. Once you get the hang of using an SPL Meter (Sound Pressure Level) to do frequency sweeps and plot the graphs you may find moving the sub a few inches / feet might fix 90% of your problems.
Buying a BFD can be like opening a can of worms IMHO. Although relatively easy to use they can become an obsession, forever fiddling and tweaking just because you might not be getting 100% from your system.
I am glad i bought one because if you are unsure of what you should be getting from your sustem it will only cost £60ish plus cables to find out for sure. To me this is only a little money to spend for at the very least, peace of mind, and at the very best a significant improvement to your system (although a lot of time spent getting there).
I would take it one step at a time and only get a BFD if you have no other option