I know this is an old thread but…
You do not need to replace like for like when it comes to reverb tanks but you do need to make sure it’s compatible. The coding in the previous post gives the info you need. The most important characters are the two letters which, in this case, are AB.
I can’t understand why Randall installed this particular unit in their amps because 6-spring tanks are recommended for organs and pedal steel guitar. If the point of MTS is to replicate classic amps it would make more sense to use a 4-spring unit as found in classic Fender amps.
It is entirely possible - and fairly simple - to switch units, which is what I’ve done. The number change is from the 9 to a 4 and the decay on the Fender units is 3 not 2. All other letters/digits remain the same.
The Fender-style unit I’ve gone for is 4AB3C1B. But you could use a 4AB2C1B too.
The only work involves unscrewing the tank carefully and removing the PCB and re-attaching it to the new unit. While I was at it I also replaced the fan with a decent 80mm computer fan (Nanoxia, same as I used on my RM100). Oh, and the G12T-75 is being replaced with a Vintage 30.
Cheers!