I've been playing around converting old tube gear to guitar amps for a little over a year and am hardly an authority, but here's my 2 cents anyway. 1) I think you'll be disappointed in the sound using your KEF drivers unless they truely produce the bandwidth guitar amps are engineered for. 2) It would be a mistake IMHO to do anything to the Dynaco to get a "guitar" amp sound. It's overkill and would destroy a great Hi Fi amp. However, if you simply intend to leave it as-is and plug a pedal into it using a 1/4" to RCA adapter cable, what do you have to lose?
I've found that old Tube PA amps are much easier to mod for guitar use than hi-fi amps. If you want to build a guitar amp from old Tube hardware, there's an abundance of old Tube PA amps out there cheap. They are easy to convert for guitar use and take pedals very nicely. One that I've had good success with is a Bogen CHB-50 - they can be found for $50-$75 on ebay depending on condition. 50 watts output with 2-6l6 tubes. They usually need a recap job and 1/4" jacks, and may or may not need new tubes, and they're ready to go. In this stock condition they tend to be voiced fairly neutral - near zero breakup. The Bogen CHB-35 is also easy to find, and easier to convert, but uses 7868 tubes which are less common. Still, it puts out 35 watts, and is otherwise the same as the CHB-50.
I've found that old Tube PA amps are much easier to mod for guitar use than hi-fi amps. If you want to build a guitar amp from old Tube hardware, there's an abundance of old Tube PA amps out there cheap. They are easy to convert for guitar use and take pedals very nicely. One that I've had good success with is a Bogen CHB-50 - they can be found for $50-$75 on ebay depending on condition. 50 watts output with 2-6l6 tubes. They usually need a recap job and 1/4" jacks, and may or may not need new tubes, and they're ready to go. In this stock condition they tend to be voiced fairly neutral - near zero breakup. The Bogen CHB-35 is also easy to find, and easier to convert, but uses 7868 tubes which are less common. Still, it puts out 35 watts, and is otherwise the same as the CHB-50.