The majority of suggestions for a guitar reamp box is to use a transformer like in this classic example.
❔ Is there a reason that op amps unity gain or other are not typically suggested?
It is my very top level understanding that "ideally" opamps have a near infinite input impedance and near zero output impedance and that placing a resistor in parallel near the input and output, one could effectively set an impedance of their choosing. If this is accurate, couldn't one just place a parallel resistor R1 in front of the opamp that is 10 times greater than the output of their audio interface and then on the output place a 10k resistor on the output side R4 to match what an EMG 80/81 is?
I am aware I am missing some capacitors to block dc, etc in this crude drawing.
Using a transformer has the benefits of having a passive device and not need to worry about a power source.
But if you intended to designing an "active" reamp box that could have additional features such a analog/digital VU meter, etc, the power source would already be there.
Transformers are used for isolation (either for safety or break a possible ground-loop and eliminate hum). The opamp circuit is not isolated.