output impedance

I'm looking at replacing my Topping A50s with the SP400 as a headphone amp/preamp to my speaker amp. I'm curious to know if the input impedance stated by SMSL is real because my DAC (Denafrips Ares II) has oddly high output impedance which doesn't play well with the A50s' strangely low input...

So I'm running the AHB2 and I'm pretty happy with it. However, it has a relatively high output impedance: The 23 mOhm at low frequency are not too bad, but it rolls of quickly above 500 Hz. For the speaker cable I'm using a high frequency litz wire with 1050 strands of 0.1mm wire (8.24mm^2)...