JimL11
1000+ Head-Fier
Well, I can tell you that the Stax amps sound different, even to my untrained ears. I have not heard 727, but currently own 252S, SRM-1 Mk2 Pro, and 353X simultaneously. And while 252 isn't bad by any measure, between those three models at least, you get what you pay for and each step up is better to my ears, and in the order listed.
I fully expect BHSE and the KG energisers to sound substantially better still, but this poor boy will never be able to afford them ;p 353X is the pinnacle of what I am able to spend..
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.