I have one of the stepped attentuator kits s Bob sells and have been using my newly rebuilt ST70 without a preamp. I imagine it would work well in your case. See here:
About 1/2 way down. In the full clockwise position, it's a straight pass through.
The only downside is that it takes the place of the stereo/mono switch, and if it's a stock ST 70, that would need to be removed and bypassed, and I don't know if there is enough room, I put mine in with the rebuild and a VTA board. I'm not sure if you are using the amp mono or stereo for the Leslie (I think Stereo based on your previous discussion). Also, you could probably mount it remotely in a seperate box and use RCA connectors.