I developed a bias tremolo system for my tweed Bassman and like it so much that I'm offering a kit version.
It's suitable for any amp with a LTPI. The whole story is at Amp-Aid.

As far as I know, (let me know if I'm wrong) this approach is an entirely new bias tremolo system. There are no changes to the amp signal path and only one connection is added from my circuit to the PI (in addition to 2 power connections). It's really easy to add to an amp and it sounds wonderful (IMO). You be the judge - there are sound clips posted.