The ST-70 has long been considered the better sounding amp.
The important thing is to make sure its power supplies (including the bias supply and its rectifier) are operating properly. The often need resistors on the circuit board replaced too. Nice coupling caps really help it out; try the V-Cap ODAM part. You can replace the rare and expensive 7199 with a 6GH8A (which gives you lower distortion) using this adapter.