Dynaco ST70 is a classic that can be had for under $400. Look for an upgraded one on e-bay. The push-pull amps of this type are all pretty similar, the design dates back to the 60s (50s, 40s???), despite the high prices at the high end stores for fancy cabinets.

If you can build it yourself, and you can, try Bottlehead for an SET amp. Much better sounding than push-pull, but low power.