I would like to clarify a few things here.
Using a passive attenuator (like the HLabs ones) on the input simply reduces the level of the input signal.
This will give you more range on the A-100's Volume control (if you find yourself only able to use the A-100 with the Volume control very near the minimum setting).
(This will give you more range on the knob because, by reducing the level of the input signal, you will be able to turn the A-100 up further.)
(And, because potentiometers like the one used in the A-100 often don't track channels well at extremely low settings, it will also help eliminate channel imbalance issues you might have.)
It will also reduce the noise floor - if and only if the noise floor is being raised by the noise floor of your source component.
(You are reducing the entire signal coming from the source - including its noise floor.)
In theory it should not affect much of anything else....
The little gadget from Amazon seems to be putting a series resistor in series with the output of the A-100 (from their description).
This increases the output impedance of the A-100 as seen by the headphones.
The dropping resistors in the A-100, which do exactly that, are 220 Ohms.....
Therefore, if you put in the jumpers on the A-100, then add the 220 Ohm version of this gadget, you will be back where you started ..... you have replaced the 220 Ohm resistor in the A-100 with an external one.
If you use it without putting the jumpers in the A-100, then you will have doubled the series resistance to 440 Ohms ..... this should result in a lower output level and increased interaction between the A-100 and your headphones.
With the jumpers installed in the A-100, the output impedance of the A-100 is a fraction of an Ohm, so, if you use one of these devices, its impedance will become "the output impedance".
With the jumpers NOT installed, the output impedance of the A-100 is 220 Ohms, so, if you use one of these devices, "the output impedance" will be the impedance of the device ADDED TO THE 220 OHMS INSIDE THE A-100.
These explanations are based on the description - which suggests that the little gadget is simply inserting a series dropping resistor.
(If their explanation is incorrect, and they are using a two-resistor voltage divider, the result would be similar - but the actual values will be somewhat different.)
Note that, as a generality, the higher the series resistor, the more the impedance of your headphones will interact with it.
This means that:
1) a larger resistor value will cause a more significant change in sound due to its interaction with the impedance of your headphones (this effect will be different depending on the particular headphones you have)
2) as you use larger resistor values, the difference in the change you get from its interaction with various different headphones will also probably be greater
Yes but you negate the advantage of the higher current swing. You could employ resistors in the signal chain, but why? It was one of the things for me too the A-100 noise floor in bypass. Maybe others may have more but I think load resistors are the way to go.
I ordered these
www.amazon.com/gp/product/B07FMGTBBL/I found out later these also use a resistor. So I don't know if it would be different from just using the droppign resistor in the mini-x. I get to choose between 80 ohms and 220 ohms.
I also recently tried a harrison labs passive attenuator 12 db rca. That appeared to reduce the noise a bit (but not greatly). Though the sound felt cleaner, it felt like it lacked upper end air. The best way I can descirbe it is that I felt like the sound lacked echo or resonance like it stopped too soon.