The wet Sounds is absolutely a marine built piece of equipment. EQ's and head-units tend to be more out in the open, so going with a marine unit with a conformal coated PCB has its advantages, but it goes far beyond that.

The WS-420SQ has chassis mounted RCA jacks on the back. the others have the RCA's soldered to the PCB and passing through the rear of the chassis.

The WS-420 has dual zone equalization, the others have single. So the 420 allows you to fine tune the in-boat and tower zones independently of each other. On the others, what ever adjustments you make to the front zone, effects the rear zone, or in-boat and tower zones if the fronts been relabeled.

The 420 has 3 independent volume zones. This is different from front/rear fade.

The 420 has 6 discrete outputs as opposed to dividing the front/rear level through the fade.

The 420's level adjustment are under the cover, so no holes in the top of the chassis where water can enter.

The 420 has true sub volume control and no adjustable low-pass filter, which is not needed with modern amps.