I can't tell exactly what value resistors are actually in the impedance adaptors, but at the risk of starting World War III (LOL!), I can tell you that without a doubt you are increasing the output impedance of the amp.
Or to put it another way, you are increasing the load impedance.
In really simple terms, it works out to the same thing, as far as you are concerned.
Easiest way to verify this is to try the 'phones with and without an impedance adaptor.
If adding an impedance adaptor decreases the volume then you are undoubtledly increasing the Output Impedance by adding the impedance adaptor from the 'phones point of view.
But the amp also sees it as an increase in Load Impedance.
Sounds like the impedance adaptor form the UERM pack is the same or very similar to Wired's "Franken Circuit". Sounds like this is the one to go with. Like Franken Circuit, the UERM adaptor has stuff in it to counter-effect the effects of high output impedance. I assume this adaptor also reduces hiss and volume?
The impedance adaptor from TF10s is probably just a resistor in series with the 'phones. I assume this reduces hiss and decreases volume?
BTW, I am an Electrical Engineer with approx. 25 years experience with Power Distribution, Power Conditioning, Power Conversion and Analog Signal Processsing.
For a reality check, I asked a few of the Electrical Power Distribution guys whom I work with if the adaptor increases the output impedance of the amp, they all said yes.