I'm interested in a Tempest design that will allow me to plug a port for low HT bass (20hz) then take out the plug for a high-out application (24hz). Here's what I have come up with... The box will be 194L net with three 4" PVC tubes- all at a length of 21.7". If all three are open- the tuning should be at 24.5 hz with a large bump (~1.2hz) around 30hz-48hz in free space. This would be very loud in that 35hz region. This should be interesting for rock music and surely fun at parties. If one of the ports are plugged- tuning would be right at 20hz which is very much like the SBB4 alignment. Does my math check out? Is there anything wrong with a design as such? Last question- what the heck do I use as a plug?