I'm not sure of the technical reason, but he said that the Tweed Circuit is Similar to a plexi circuit, and it's hard to get the circuit to balance when a reverb circuit is added. That's why Plexis and Tweeds really don't use reverb. I think modern amps can be made to work with reverb, which is why the new Vox's, Blues Devilles, and DSL's have reverb. One of the benefits of printed circuit boards.