The basic difference as I understand it is that for the same given amount of power the KT88 has more slam and tighter bass whereas the EL34 has more midrange "magic" going for it.If you get between 2-3K hours out of the tubes think about what the replacement cost will be.Go to ww.thetubestore.com to see an average price.I have a Mesa baron which uses 12 6L6 tubes one that is less associated with audio equipment than guitar amps.I have thought about getting the mods don for $500 which would allow me to use EL34's.Some amps have designs like the Rogue where you can just drop in the KT88,EL34,or 6550 (a tube a lot fo folks like as it has more dynamic slam than the EL34 but is much cheaper than the KT88).One last thing.What's a low of fun istube rolling i.e trying new tubes to get different peformnance.The new tubes are not as high a quality as older tubes made in the 50's and 60's.They are called NOS or New Old Stock (if in fact they are new).Tube were so widely used before the introducton of Solid State electronics many of the tubes from back then are still avalible.But you will find that the output or power tubes as they are alternately called are prohibitely expensive.But in all power amps you have input/driver tubes and in a intergrated amp these tubes handle the line stage (pre-amp) functions.These tubes can be 12AX7's,12AU7's etc (many web tube sites will show charts to indicate which tube is a variation another i.e. a 6922 is a version of the6DJ8).As opposed to output tubes they will last longer (up to 10K hrs.)You can experiment with telefunkens (the King),Mullards,Amperex etc to make major changes in the quality of the amps sound but getting these NOS tubes.As far as the power tubes go most of them are now manufactured in former communist countries as there military electronic stayed dependent on tubes long after the west had gone solid state.if you need to replace your power tubes one brand stands out that being Svetlana.Another better brand is Sovtek.The least expensive ones but the worst sounding are the chinese tubes.It's a matter of taste but since I listen to more jazz than rock and am a cheapskate I would pick the EL34 amp as I dig midrange.But after you have the amp come back and ask or do a search on tube types and get a set of NOS drivers no matter what amp you get.The results can be startling and tube rolling is fun.Welcome to the nutty retro world of glass audio!!!!!
Chazzbo
Chazzbo