Duke's answer, above, regarding speaker thermal and mechanical limitations was some high-quality info delivery. Nicely explained and much appreciated; thank you!
In other news, today I tried driving my Birdhaus 48 with the little Dean Markley guitar amp chassis (model K-20; 15 watts @ 4 ohms) that I used to use as a sort of bottom-feeder "Marshall plexi-esque bass preamp." But today I skipped the line out jack and power amp part of the equation and just cranked the 15W internal amp up and drove the cab directly. Sounded really good! Pretty much the same sound I have tweaked up in my normal Zoom bass preamp using its Marshall Superbass model, actually, and happily, when the little Dean Markley's post gain knob is floored, it compresses a little bit and even has a pretty similar response/feel to what I normally use. The Birdhaus is efficient enough that the 15 watts feels loud for practice--enough to rattle sheetmetal file cabinets and framed pictures--but not painfully so. It wouldn't keep up with rock drums, of course, but it could probably gig a small venue with acoustic guitars and maybe a drummer with brushes.
Since the Dean Markley has a guitar-specific 3-band passive EQ, with a bass frequency center that is probably too high for bass, I found the pluggable ports very useful: at home it sounds a little congested with the one or both of the ports plugged and I can't EQ my way around it with the onboard EQ...not enough control in the right place. But with both ports unplugged and the treble and mids floored, it sounds really great. I'm on the drive channel but with the gain down low, maybe 9 o'clock, and it's only 15 watts, so I don't have to worry about lack of speaker excursion control by running the ports open. They just serve as sort of a last-stage low-end-response shaper. Once again, variable port area to the rescue!
I wish I had some low volume or acoustic jam buddies to try it out with, but if nothing else, this will save me toting my power amp back and forth from home to practice, and I will be sure to try the Dean Markley next time I have to record some bass. Cool!