Quoted from markmon:Yea, you can put a vol control on your backbox speakers and do the same thing for way less. Then you up your volume, lower your backbox and you get more bass without more volume. Another option is to just solder a 16 ohm resistor inline to the + terminal on the first backbox speaker. This may lower the volume on them enough.
I like the idea of the vol control, never thought of that!