Finding cheap single ended valve amps to experiment with is still fairly easy, after all nearly every mains valve wireless ever made has a SE pentode amplifier with a couple of watts output. If you want something a bit beefier then you need to be a bit more adventurous. Some of the better radiograms had push-pull output, as did cine projectors.
Here are a couple of examples of projectors I've bought on ebay and the amplifiers they contain.
The Gaumont-British L516 is an early 16mm sound projector. Unless you're lucky enough to find a complete system you won't have an output transformer or power supply choke as these are part of the loud speaker unit. What you do get is a couple of output pentodes, a good quality inter-valve phase-splitter transformer and a microphone input transformer. The power-supply is probably not of much use these days 110V AC/DC with a 200mA barretter for series wired heaters. The exact valve line-up varies. I've seen two of these, one with octal valves - 2 x CL33 and EF36, another with B7 valves - 2 x PEN36C and SP13C. Other than the base CL33 and PEN36C are the same valve.
The 601 is from the late 1940s. With this you get a complete and quite compact valve amplifier using the ever popular pair of 6V6 push-pull with 6SL7 phase-splitter. Unfortunately it expects 110V AC mains. Also the downside of being compact is that the underside of the chassis is very crowded. To restore such an amplifier is quite possible, but if - like me - you simply want to turn it into a usable mono amplifier, then stripping out all the components from the underside and re-building with a 240V transformer is most likely easier.
Mine came with two metal 6V6 for the output and a glass 6V6GT as the exciter oscillator. The other valves were also glass.
I'm tempted to use this as the starting point for an all metal valve amplifier. Though the 6SL7 was never (rarely?) produced in metal form its predecessor the 6SC7 was and I have several. The circuit will need to change though, as the 6SC7 triodes share the same cathode.
As of 20th April 2009 the amp is rebuilt with a 240V transformer and a full complement of metal valves, even the 5Z4 rectifier has been replaced with a metal one. It gives the amp a somewhat military look - not quite a spy radio but heading that way. See here for an example of a Polish spy radio using all metal valves.