Q - Why does EUNA want to be first? What's up with fuzzes and wahs?
Why does EUNA want to be first?
I always recommend that EUNA be first in the chain, because the passive output of the guitar is the most sensitive part of the signal chain. The maximum benefit is gained when you plug the passive guitar straight into EUNA. Why? EUNA is a steel box (great shielding) with about an inch (very short) of heavy copper traces (excellent fidelity) connecting the guitar's passive output to the active, high fidelity amplifier stage. If you look inside a condenser mic of any reasonable quality, you'll find that the connection from the capsule to the active input of the tube (valve) or FET is lifted off the PCB. This is because the output impedance of the capsule is incredibly high (there are often 1 gigOhm resistors around it!) and at that super-high impedance even the PCB material's capacitance becomes a limiting factor in top-end response. Is the typical electric guitar's output impedance this high? No, absolutely not. But the principal works, and in most guitar rigs you can start to hear loss with just a few jacks and switches hanging on the passive signal if it doesn't have an active stage. In some cases it's not a large amount of loss and a player may choose to ignore it, but if the goal is the least amount of loss, EUNA first will be best. Since the input impedance of your typical fuzz or wah is low enough to cause loss on it's own, it's fine to run those in the EUNA loop without noticeable additional loss.
Why does OAMP want to be last?
OAMP wanting to be last is a similar principle, it is designed to drive a long cable and still hit the amp hard. Passing through any additional active devices is a potential bottleneck, but sometimes this can be ignored if you have a specific use-case.
What's up with wah and fuzz interaction with buffers and other amplifier stages?
Wahs and Ge fuzzes tend to create the tone we're expecting from them when presented with the higher output impedance of the passive guitar, that's why people often recommend one or the other first. The designs use the output impedance of the previous device (they presume a passive pickup and tone stack) directly as part of the tone shaping rather than buffering first. This is why they respond so differently with different guitars and cables, etc. Wahs and Ge fuzzes tend to also have higher output impedances than more modern designs, so this is why they interact strongly with each other when in series. I often recommend that both of these devices go into the EUNA loop in whichever order you prefer, because generally you'll want EUNA out of the way of both.
What about switchers?
I recommend EUNA in front of the switchers for the reasons listed above, but because many switchers uses really high-quality jacks and switches the loss may be small enough that you don't notice. Sometimes it gets a little academic, a single pair of high-quality jacks and one relay-based sealed switch won't make a huge sonic difference except on higher-than-usual-output-impedance guitars. The loss is definitely present and measurable, but if there is another important factor (like multi-pedal switching patches) you may choose to ignore it and that is totally fine. As the loss becomes audible, in addition to a level drop the resonant peak of the pickups will dip. If the sound is a bit pokey in the upper midrange you may want to try EUNA before the switcher and evaluate from there.