can you mod a champ to sound "british" 
 

You might get better results if you change the value of a few components  between stages. You might try changing the cathode bypass cap on the preamp stages from 25uF to 1uF or .68uF. You might also try putting a 470k resistor in parallel with a 470pF cap (after a .02uF to block the DC voltage) between the two stages of the preamp, and moving the tone stack between the preamp and the power amp.
 
I think that you might like these changes without changing tubes. EL-34's  might work with little or no change to the circuitry, but you would have  to keep your plate voltage down on the EL-84's, and re-bias your amp.
 
Hope this helps!
 
Chris B