Digital effects unit for tube amps

OK, there's no such thing as a special purpose digital effects unit for tube amps.

First I need to find a place in the amp that can work as a mixer stage, as close to the power amp as possible. Then I need to attenuate whatever the signal is at that stage down to the correct level so as not to fry the input of the effects stage. Following the effects unit I will need a gain stage to take the effects level back up to whatever it was coming in from the previous gain stage.

Thinking about using simple resistor divider on the FX input and a MOSFET gain stage post FX.

Maybe this is best done with an amp using a long-tailed phase invertor since that can act as a mixer stage.

Speaking of the effects board itself I am considering the Spin FV-1:

http://www.spinsemi.com/knowledge_base/demo_board.html

Here's a page describing this board used in a guitar amp. I would prefer not to run the entire signal through it - just the effects send, keeping the dry signal intact.

http://www.spinsemi.com/guitar_amp_application.html