The sonic differences are VERY subtle. Some people hear them, some people don’t. The audio portion on the circuit is identical. What is different is the power supply.
The original units and our FET/RACK use a really robust over powered supply. The FET/500 uses the supply from the 500 series rack, which tend to be more modern and efficient. If you have great ears the difference in the power supplies may be noticeable, but I find them to be pretty identical.
The 500 series are small, have a 2:1 ratio, and true bypass which is nice. The Rack units are 100% authentic and are super cool.
Really comes down to what’s important to you.