Shreekant,
Nope, can't use a 6sn7 -- the 6dn7 is half signal tube and the other half, Power tube -- dissimiliar triodes.
The 6em7 can be used in the s.e.x. but requires some resistor and other changes if I recall correctly. Sorry, but you need 6dn7s, so pick up some spares (though they will last a very, very long time in this amp.)
HTH,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)