There is very little difference in tonal qualities for the Wal basses. They all feature the same circuitry that gives them their signature growl. The differences actually lay in the appearances between the MK 1, 2, and 3 bodies.
The MK 1 features short, stubby horns and a flat back end. They closely resemble a Fender Precision bass that has been squished bridge to neck.
The MK 2 features a much longer top horn but is generally the same shape as a 1.
The MK 3 features the longer horn of the MK 2 but also adds in a slanted back end behind the bridge, running from the strap peg inward and upward towards the players chest. The photo for this entry is accurate as being an MK 3 Wal.