Leave the Zoom's input gain adjust at default (100, if it's the same as my old H2) -- the H2's numeric adjustment just scaled the already-digitised signal, which was no good and resulted in a loss of bit depth.
I wouldn't use the Mic In unless it doubles as a Line In -- or just use the XLR Ins and set them to Line Level if possible.
The Mic In has a lower input impedance - the more you boost a circuit (like a mic preamp circuit) the noisier the signal gets, and the higher the impedance the more noise there is to begin wtih. You should preferably be using the TRS ins if you have TRS out of a mixer.
Impedance describes the relationship of voltage and current. Remember Ohm's and Kirchoff's Laws?
Good quick read: https://electronics.stackexchange.com/a/21788/72394
Pinched from Whirlwind's web site:
Impedance (Z) is the measure of the total opposition to current flow in an alternating current circuit. It is made up of the sum of two components, resistance (R) and reactance (X).
In audio we talk about output and input impedances. Output impedance is effectively the internal impedance (resistance) of the circuit as measured across its output. Input impedance is the impedance as 'seen' by anything connected across a device's input.
Having balanced ('matched') output and input impedances used to always be done to establish optimum power transfer, but nowadays it's not desirable in audio environments. (there's a few places where impedance matching is still done, but it's usually with very old or niche equipment like ribbon mics and their preamps).
Where high (or mismatched) impedances can cause issues is in long cable runs: you can experience 'reflections' (quite literally part of the signal bounces back, mingles with the outgoing signal and causes spikes, interference etc) and it reduces the overall signal level.
It's a pain with digital signals like AES3, it falls off a cliff pretty quick once the circuitry can't distinguish the edges of 0s and 1s.
It's why having low impedance cables can be A Good Thing.
Impedance is all about voltage - but we must consider current and a circuit's resistance. NB: impedance is not uniform across the frequency spectrum; you can have varying impedances at different frequencies but audio equipment's designed to be uniform across its operating range.
Unless you have an existing electronics education or background, this next bit's a little counter-intuitive at first.
(I don't claim to have anything past a functional understanding of essentials, what I've learned in my job and read when studying.)
If you have a mixer and a recorder, having a high input impedance on the recorder will result in a higher voltage (so higher signal level, and larger signal to noise ratio) across its input, which is good - a high input resistance prevents current from passing through too easily and the output supplying the signal can more easily supply a good voltage level to the input circuit.
If the input impedance to the matched the output impedance from the mixer, you would see input signal level reduction if you hooked up more than one device in parallel to the output. Parallel resistance is a bit weirder than series resistance.
Impedance bridging, or 'voltage matching' (where the mixer's output impedance is lower than the input impedance of the device connected to the output by at least a factor of 10) is the norm in pro audio. It allows for the full output voltage to 'develop' across the input impedance and requires a higher output voltage - so less current draw - resulting in less distortion and better frequency response.
That Whirlwind page has a great hosepipe analogy (an electronics teacher explained it similarly to me, using pipe diameters in a central heating system):
Think of this as having a nozzle at the end of a garden hose. The garden hose is a low impedance source (there is little resistance to the flow of water) and the nozzle is the higher impedance of the input being fed by the hose.
When the nozzle valve is closed (open circuit):
- Input impedance is VERY high
- Pressure (voltage) is at maximum
- Flow (current) is zero
Now open the nozzle just a little:
- Input impedance reduces but remains high
- Pressure reduces but remains high
- Flow is small
As you continue to open up the nozzle:
- Input impedance reduces further
- Pressure reduces
- Flow increases
With the nozzle open all the way:
- Input impedance is very low
- Pressure falls dramatically
- Flow is greatest
In the case of a high impedance guitar output (7,000 to 15,000 Ohms or more) driving a relatively low impedance input of a mixer (2,000 to 10,000 Ohms), it's like connecting a garden hose to a fire nozzle. The hose just can't produce enough flow (current) for the size of the opening (impedance) to maintain the pressure (voltage).
Some further reading... It's worth at least getting your head around the concept of impedances because we all come across it so often in audioland.
Hi-Z outputs on guitars are just high output impedance :) Which is why DI boxes are used, to reduce the signal to a low impedance for feeding into a line level mixer input (and converts it to a balanced signal, which inhibits interference).