Well, I went to my local sewing shop and got a bag of synthetic wadding of the kind used to stuff sofa cushions. I carefully unscrewed the speaker from the Rumble 15 cabinet, and stuffed about a third of the contents of the bag into it. After reassembly, I fired up the Rumble, and lo and behold, that terrible boxy sound was gone. A little bass bost, and it sounded quite tight and nice. Of course, it's still a 15 watt combo with an 8" driver, but this improved the sound no end!Great! Thanks for the tip! Will let you all know how it goes.