I ordered a Weber "Blue Pup" AlNiCo 15-watt eight-inch driver for my '15R to replace the stock ceramic-magnet speaker. The replacement went nominally well except the original driver must have been installed while the paint on the cab inside was still tacky; it was a fussy job to get the original speaker off without messing up the ring gasket. This is the first time I've had this amp apart. The electronics design and workmanship are excellent; very, very clean. There is even a ferrite toroid at the point the speaker wire exits the chassis. This is excellent protection against common-mode interference from things like radio transmitters, dirty old florescent light ballasts, leaky neon signs, et-cetera. Even an Ampeg SVT does not incorporate this excellent feature.
After I put it all back together I played the new speaker at moderate volume for about twenty minutes. Compared to the stock speaker, the Weber is noticeably livelier; somewhat louder, and much more touch sensitive which is to be expected with the very lightweight moving parts and the AlNiCo magnet. It will be interesting to hear how this speaker settles in over the next few weeks. Thus far I am highly pleased, and glad I went to the tiny trouble involved.