Which tubes to roll

Thanks. I noticed in the manual it says that the following tubes can be substituted for the KT88s: 6L6, 6CA7, KT-66, 6550C, KT-90, KT-99 or EL-34.

Just reading another thread and the poster likes the Mullard EL-34s much better than the KT88s. I notice the 34s are less expensive. What, in general, is the difference between tubes that can be substituted for each other?

The big difference between the tubes you mention is "power dissipation", or the maximum output that the tube is designed to produce at a certain plate voltage. ie: The average KT-88 will be designed to output about 35W, while the EL-34 is more between 17 and 20W. The Telefunken EL-34 was designed to output about 25W, which is high for it's type. The KT-90 and EL-34 tubes are at the opposite ends of the power output spectrum in your list, with the others falling between. There are also differences between tubes within one type, ie: the original 6L6 was a 19w tube while the 6L6GC is usually rated at 30w. A lot depends on the plate voltage the tube is manufactured to handle, and whether it's a tetrode or pentode being run as such or as a triode. Even a pair of EL-34s can output 90w with 800V on their plates. The differences in presentation/sound between all those tubes would be ridiculous to even approach, given the multitude of variables. ie: Manufacturers, amp designs, variations in bias/plate voltage, again- are they being run in triode mode, etc. There are also a plethora of personal tastes involved in the analysis of tube "sound". Again- Kevin has a vast amount of experience with various equipment and tube designs. Tell him your tastes and he can direct you to the tubes that can best satisfy them.

I found a good website with an explanation of the evolution of the these tube types, the hows, whys, and what-nots.

http://www.jacmusic.com/KT88/kt88.htm

That's a good link(thanks)!! His observations on chinese business ethics(there's a stretch) and manufacturing, mirror mine. BTW: If you like transparency, nice freq. extension in both directions, and liquid mids- The Winged 'C' 6550C's(latest iteration/St Petersburg plant) that Kevin sells are working very nicely in my Cary monoblocks. I was a hardcore NOS tube addict, until the cost of matched quads/octets of anything decent in power pentodes became unrealistic(Dom Perignon taste/grain alcohol budget). Uncle Kev came through, in style. You could say I'm a happy customer. Does it show? (http://www.upscaleaudio.com/SED-quotWinged-Cquot-6550C_p_16.html)

Tubemonger.com is a nice organization to work with. They may not have the rare and hard to find, but their webpage and service are customer friendly.