One thing I'd do differently is to boost the bass to match the mids. The Altec midwoofer is front loaded and reflex tuned. The front horn is tuned for midrange, which has the effect of introducing a midrange shelf that's louder than the bass. I'd pad the midrange down with a bypass coil to boost the bass. On one hand, that lowers system sensitivity but on the other hand, the midhorn load will still reduce midrange excursion. That means the benefit is still there, in the form of reduced midrange distortion. Don't push a system like this hard though, because bass excursion will tend to modulate the midrange, creating intermodulation distortion. Since you're talking retro style and tube amps, I don't think this will be an issue. It will probably be pretty cool. Keep us posted if you decide to do it!