There is actually a pretty straightforward reason why those three amps sound progressively better. The 252S circuit is reportedly similar to the 323/353 circuit, but runs at a much lower output current, so if you have a tendency to crank it, it begins to run out of power. The SRM-1 Mk2 Pro circuit is somewhat different, with a similar output stage standing current to the 323/353, but uses resistors for the output load rather than the current sources used in the 252 and 323/353. This means a significant portion of the output current is wasted driving the resistors and is not available to drive the headphones, so it current limits sooner than the 323/353, and also generates more distortion as the output devices have to work harder. The 727 in stock form suffers from its non-global feedback output stage, resulting in uncontrolled bass, among other problems - see the review in InnerFidelity for more details. Modifying the amp to add the global feedback improves the sound significantly and is easy to do but does void the warranty. I haven't heard the 323/353 so can't comment compared to the 727. I personally prefer my refurbed and modified SRM-T1 with constant current output loads, but that's just me.