I have a collection of old Fender amps, and recently played through them; four out of five had marginal sounding reverb, leading me to conclude that spring reverbs have a certain lifespan, and eventually start to diminish in duration and tone. Thinking about it, how long can a set of small springs stay in their original condition, being subjected to countless moves, temperature changes, humidity etc?
So I dug deep into reverb tank research, and found out there is basically two types of tone:
1. the Accutronics surf-rock sound which has a very metallic white noise at high settings, like a reverb with hints of a bright ride cymbal
or
2. the TAD or MOD reverb tone, which is (to my ears) more lush, round and musical, more FLOYD-like.
As many of you know, replacing the reverb tank on a Fender amp is THE EASIEST thing you can do to improve your amp sound, and the cheapest; usually around 40 bucks, and as easy as unplugging two RCA jacks and replugging into the new.

The only trick and essential thing is to get the exact one matching the impedance of your specific amp, and there’s a bunch.

There are also other considerations: a short or long tank, two or three springs in the tank, and more importantly, the duration of the reverb you want, meaning how long your reverb sustains.
You can buy short, medium or long.
Anyway, bottom line: Cheap and crazy simple to do, and HUGE DIFFERENCE IN SOUND QUALITY!!!
Try it!