This one is from the 60's, they stopped making the SRO alnico in 1968 I have read, but I they made the ceramic version in the 70's. These were in many Twin Reverbs. Jerry Garcia was known to use the SRO's before going to JBL E120 speakers and other JBL's. I have a EV 12L in a Fender Blues Deluxe, another great improvement. You can find EV 12L varieties on the interweb, SRO's a little harder to come by, and they will run $150 - 300 depending on the day and seller. Valuable, I don't know but the price keeps going up. I would not go for the Zak Wylde 300Watt based on what I have read. Other similar options I looked at were the Weber California and older JBL 120's (E and K), and don't forget EV is selling the 12L again. Mesa Boogie has a history with EV and you can look up the model letters/numbers and see exactly what speaker is used.
I specifically went for the low breakup in these two amps. Other amps for more breakup. After a few months I can say this speaker is still a huge improvement over the standard Fender speaker. Louder yes, but more musical and fuller. like you said "presence and clarity" is a good description As always YMMV.