No offense Grendle but this isn't exactly correct...
The USA 9v and USA 18v circuits are BOTH made by HazLabs... The ReBop has had many different circuits through the years but comes standard with the Czech-made TonePump now.
Regarding the USA circuits... I have both, and have compared them side-by-side.
The output on the 18v IS hotter than the 9v. This surprised me the first time I switched them out on the same amp.
The difference with the knobs is a minor thing... the three knobs of the 18v model have a stacked bass/treble knob so in reality it's pretty much the same setup as the USA 9v.
One knob for overall volume, one knob for blend, one for bass and one for treble.
The main thing is that the 9v and 18v USA Circuits are based on the older op-amp system which is known to loose signal when processing a lot of bandwidth from the pups. The reason the 9v USA Circuit is known for so much growl is because HazLab's and Stuart's (remember the HazLab/SPECTOR circuits are made to Stuart's specifications) solution for the signal loss was to add additional op-amp circuits and distort the overloaded signal... distort meaning "distortion". Which is the source of the growl.
Different manufacturers handle the op-amp signal loss problem differently... but the most common method for addressing this known issue is to add more voltage to the circuit, reducing the resistance.
The 18v USA Circuit does just that. And as a result the extra headroom provided by the additional voltage, creates a cleaner, less distorted sound. Not to say you can't get the 18v circuit to sound "growly" because you can... but you can also clean up the sound and get a clearer tone if you want.
I have to say that I prefer the sound of the 9v circuit better... but honestly I think it's because I prefer the P/J setup to dual humbuckers. There's more tonal range... it's almost a 3D sound effect from the P/J setup whereas the dual humbuckers tend to sound more alike (given their positions under the strings).
If I get time after BPL I'll record some samples with both for you.