What is the input/output impedance of EUNA/OAMP? Is 1M/100R the ideal spec?

EUNA is not technically a Class-A device although it is biased using a technique that creates a Class-A response. It's a common misconception that Class-A means "better," Class-A is just the oldest type of amplifier circuit. Depending on the particulars a Class AB device like EUNA can have a much better frequency response, much higher input impedance and much lower output impedance. EUNA's input impedance is over 2 MegOhms and the output impedance is less than 100 ohms, closer in reality to 10 Ohms. There is a popular video telling you to look for 1M input/100R output, but sadly there's some misinformation there. 1M input/100R output are commonly quoted specs from manufacturers because they are ballpark figures. Often the 1M spec comes from looking at the input load resistor, or bias resistor, which is a handy stand-in but not exactly the correct value.Most manufacturers do not calculate or measure the actual values but use these specs as stand-ins. They are generally thought to be "close enough" to the real spec. Impedance is also frequency dependent, so no single figure can actually tell you what's going on. Beware of anyone who tells you there are magic values!

OAMP has a lower input impedance than EUNA, because it's designed to be after other impedance-lowering devices in your pedalboard. Most amplifiers convert the impedance to a certain degree, so at the end of your chain the output impedance of the last pedal before OAMP is likely to be much lower than your standard passive guitar output. OAMP's input impedance is a little less than 1M depending on frequency, which I feel is superior to a higher impedance at that stage in the pedalboard. If you're interested in this subject, you can read about the shift from impedance matching to impedance bridging in detail. The short answer is that matched impedance was once the standard, so that 600 ohms output would feed 600 ohms input, which is really great sounding but has some issues. It doesn't account for impedance incurred along the way (between the two devices), but more importantly if there is a mismatch in the wrong direction (higher output impedance than input impedance) you'll get a lot of loss very quickly. Bridging impedance is a great fix, meaning the output impedance should be as low as possible and the input impedance should be sufficiently high to avoid loss. Very high input impedance causes its own issues, so it shouldn't be higher than needed in any given application. EUNA's input impedance is very high, for instance, because the passive output of the guitar is high, and we want to be certain there is no loss. OAMP's input impedance could be considered medium or medium high, because it is designed to be fed by an output impedance between 100 Ohms and maybe 1K ohms, which would be on the higher side of what a typical pedal would put out. OAMP is still sufficiently high to plug a guitar straight in, but depending on the guitar it's not as well positioned as EUNA to prevent any audible loss.