An interesting thread indeed!
While there is a lot to be said for the old Ampeg B-15's and vacuum tube equipment in general, I vividly remember the realities of owning and using those amps. A friend, who was already a very accomplished jazz player when I was just starting out, used to play his beautiful Prescott through a B-15, and it often sounded pretty bad -- real boomy and rumbly. I heard him playing last week through a rather modest modern solid state amp, and he sounded absolutely amazing. (OTOH I also remember seeing the Lemon Pipers in 1967, before the bubblegum days, and their bass player was using 2 Ampeg B-18N's! What an incredible sound.)
Another thing was the high maintenance, which is something that I don't think most bass players today would be willing to put up with. B-15's, like most other tube gear, were not nearly as reliable and trouble free as amps today are overall. When Polytones and other new solid state amps came along, many bass players were glad to give up their old Ampegs for something that was lighter and easier to move, had far fewer maintenance issues, and, in many cases, sounded better.
I have owned several B-15's over the years and kept the best one for many years. However I sold it a few years ago, since I just wasn't using it and it was just collecting dust. Although I have some very fond memories of those days, I don't miss the amp at all.
I also found the speaker magnet info in this thread interesting... Before the mid-to-late 40's almost all speakers were electrodynamic types (AKA field coil speakers). These had no permanent magnets, but instead had an electromagnet that required an external DC power source. In practice, the field coil was often a part of the associated amplifier's power supply.
A little info from the Audio Heritage web site: "...Since the mid 1940's, almost all loudspeakers have utilized permanent magnets to energize loudspeaker motors. This technology was the result of the war effort, during WWII, that saw the development of new permanent magnets with sufficient strength to generate high flux densities. Prior to this, the only practical way of generating the high magnetic energies needed for loudspeakers was with field coils. These required an external power source to keep the field coil energized which posed cost, operational and maintenance problems. For these reasons, field coils speakers were quickly supplanted with permanent magnet loudspeakers when the new magnet technologies became available."
I'm not 100% sure, but I think that the very first Ampeg amps used electrodynamic speakers. Maybe someone else will know for sure.
Electrodynamic speakers were replaced by permanent magnet speakers almost exclusively with ALNICO magnets. Ceramic magnets came into common use later on OEM speakers, since they were less expensive to manufacture.
BTW the Altec 421A had a ceramic magnet. See:
http://www.lansingheritage.org/images/altec/specs/components/421a/page2.jpg
The earlier versions of the Altec 418 15-inch MI speakers employed an ALNICO magnet, but the 418 usually was not used for bass applications.
Having owned and used all the classic premium 15-inch speakers (Altec, JBL, Gauss, etc.) over the years, my personal favorite is the EVM-15B Series II. I still have one mounted in a Bag End cabinet. (It even sounds amazingly good for DB.) That cabinet combined with an older GK 400RB is my modern version of a B-15 , and, to me, sounds far better than an old B-15 ever could. Not to mention it is almost entirely trouble free.
Only my opinion of course.
I hope to have more comments on it here soon. 
