Granted, there are not thousands of ways to amplify a signal. Circuits change when designers have the insight to use a SOTA device in a new way. In tube design it would be utilizing a new tube. The circuit topology has to change to take advantage of it. True you have less design options wih tube products then transistor. Bipolar transistors give way to j-fets at preamp inputs thereby lessening noise. Some of this is semantics. What is your definition of a circuit topology change? Are you aware of the fact that the way you lay the circuit board out, the circuit board material, and the thickness and width of the traces can make or break a potential design. It is true that if a designer finds that a particular circuit in a product like a power supply sounds particularly good, (yes power supplies do have a strong effect on an audio circuit), they might use it in another design. You have to remember that it's the whole package, not just individual parts or circuit topology that make up the sound of the unit your listening to. Some manufacturers actually remove or update parts as they become available after the first design because it sounds better or gives better reliability. That's not being cheap, that's improving performance and that would be a circuit topology change. In fact a new manufacturer of amplifiers has just come out with a new output transistor circuit that doesn't utilize emitter resistors. That to me is a new circuit design. I don't think someone like Nelson Pass or John Curl would say there had been little change in line stage design over the past 40 years. When new parts come out, circuits change. Everyone is entitled to their opinion.