How old is the amp? I've had bad tubes before that have caused that distorted sound on low notes too. I'd not only check the speaker, but the tube too.
Before buying a speaker, I'd check out www.valvetronix.net. Also, one thought would be if you could run the amp through another cab, that would rule out whether or not it's the speaker. Is there a line out that you could test it on another speaker cab or even through an amp's line in? That would tell you if it's the speaker or not.
Guitars: 2003 and 2004 American series strats, Squier Classic Vibe 50's Strat, Squier Deluxe Strat.
Amps: Line 6 Spider IV 120, Vox AD50VT 212, and Peavey Transtube Bandit 112.
Pedals: Digitech Bad Monkey.