Hmm. I think we need more info to help diagnose your problems.
What occurs to me is that it's possible that when blasting it you're clipping the tube amp, which generally exhibit soft clipping characteristics closer to compression than obvious/gross distortion. It will shatter the illusion if you're clipping the amp.
The Hsu high pass filter you mention is likely to protect the ported sub from excessive cone excursions below tuning frequencies (where the driver un-couples from the port output). From your description of events, that doesn't sound like it would be a solution. It is probably a good idea to protect your subs from teenager spls and hip hop, though.
How high are you running the sub's low pass filter? Perhaps there is too much overlap between your subs and mains. Subs, properly balanced with the mains, should enhance imaging, not detract from it.
Regarding compression, if that's in fact whats going on, you may get better results by implementing bass management, which would provide slightly more dynamic range, key word being "slightly". What are you working with as far as the pre-amp goes? Does it have bass management?