"Do you have to play at stage volumes to take advantage of the power valves and their different characterstics?" AngryGoldfish
That depends on the amp being used.
For example, the Einstein was not designed for power tube saturation. It may get some when run really hard, but I'm not sure.
On the other hand, the VH4 was intentionally designed to be able to get power tube saturation when the global volume control is above 1 o'clock, no matter what the volume level is.
The reason I like this feature is because it allows you to get either the preamp tubes alone, the power tubes alone, or in combination with each other, the various saturated tones you want to hear at ANY volume level.
The Schmidt can also do that, but more noticably at lower volumes with the 15 watt power tubes. I use the stock tubes that Peter placed in the amp from Bad Steben, which I believe are some type of 6L6. Since I run the ch one volume up to 3 o'clock, I can get that power tube saturation nicely and still hear the clean side of the amp on ch one.
Since the Schmidt has no global volume control as the VH4 has, getting power tube saturation only comes at a louder volume on ch one, but is more easily obtainable on ch's 2 and 3 due to those channels having a master volume to work with.
It is always a trade off depending on the amp.
I bought every single Diezel amp unseen, unplayed and have never been even slightly disappointed buying them this way.
As for blending power tubes, I change out the preamp tubes after a year of playing and try some NOS Mullards, Telefunkens, Mazda's, RFT's, Brimars, etc to try to revoice the amp, but I prefer a KT77 as my power tube of choice, thank's to Olaf's reccomendation here, as it combines the best of a 6L6 and an EL-34 from which it is originally based.
I had a blend of EL-34's in my Herbert with a touch of 6L6 in sockets 2 and 5 and the amp was wonderful, but I feel that a KT77 sounds very similar to this combination...just 130 watts less powerful.
If you have your mind set on a 100 watt Einstein, better to find a dealer or person near you that possibly may allow you to at least try it out for a few minutes to see if you actually will need 100 watts. If your intention is to use it for recording, you will never need that much horsepower. Your speaker cab will make a bigger difference at lower volume levels using a 7 string guitar or drop-tuning, or even playing at garage levels. The 50 watt Einstein is really LOUD to begin with.
Good luck, and good hunting.
Stephen