Actually, it's not as bad as you thought. I'm really bored and was curious as to how this would look so I plotted it in a 3 cubic foot box tuned to 28Hz with a random 12" sub (same for both tests) The line in red is the transient response when given full signal at all frequencies. The one in yellow is with a highpass (subsonic if you want to call it that) placed at 30Hz. It's really not all that bad.
It would work out alright it looks like. Infact,the filter smooths out the peak of the box, but it just doesn't extend
quite as low. However, it would still be more than practical for any daily musical use. I'm gonna go shoot myself now because I actually took the time to do this and I realize how much time I spend on CA.com //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif