Please don't publish plot from other amps here, to speculate about the FR of STA-9, many users are not technical and could be confused.
Regardless of the reasons, we don't recommend STA-9 with LS50. Again, the same reviewer who has LS50 recommended IDA-16 product of the year award. So this is about matching the sound characteristic that we have intentionally tuned.
Or put it differently, STA-9 was internationally tuned to be on the warm side, whereas all other NuPrime amps are neutral. NuPrime is probably the only class-D/hybrid amp company that has many different designs in-house instead of tuning some off-the-shelf modules.
If we are using someone else amp module (IcePower, Hypex for example), we will be limited to changing power supply or perhaps some opamp and capacitors for sound tuning. I asked our chief engineer why he come up with a new design for every new model. The answer is that we have a specific usage for each model, so it make sense to do so. In the NuForce days, all we had was the same fundamental design that evolved over 10 years. In the early days of Class-D amp evolution, engineers were still learning how to control and improve upon a design. Think about how car's engine evolve over the years.
It is interesting when you compare with cars. however advance is the gas powered engine, it will soon be surpassed by electric motor. Soon most of the electric family cars will accelerate faster than a million dollar sports car. Can we take class-D/hybrid design to the next level? I am sure about this