Slash v21) Significantly improved thermal performance (50% longer time to thermal than Slash v1). This is largely due to a new mechanical design with a monolithic fin and transistor mounting surface (one big cast part replaces four cast fin modules and one extruded bar on a 500/1, for example). A new clamping method for the transistors also helps improve thermal performance and reliability.
2) The 250/1v2 will no longer have a preamp output. It will have a jack for the RBC-1 remote bass knob, which the v1 lacked.
3) The 500/5 will be replaced by the 550/5v2 which is 75x4 + 250x1.
4) The v2's are slightly shorter in height than v1's (by 0.1 inches) but otherwise have the same dimensions and mounting hole pattern.
5) The v2's incorporate stacking functionality into the end cap design. Stacking will be accomplished with separately sold hardware kits specifically designed for twin or triple stacks.
6) A multitude of small tweaks to circuitry in several models aimed at improving performance and reliability.
7) Revised cosmetics and packaging
Other than that, they are just like Slash v1's.