Or you could just use another poweramp that you know is 47kohm or higher, and if that then gives you the bass your missing you know the output coupling cap is not big enough in the preamp when it’s with the 10kohm McCormack amp.
You also asked why they make such low impedance amps, sometimes it’s because they’ve use Bipolar input transistor, if they used Fet transistors on the input it can be much higher, even 100-200kohm.
That was the nice guy in me talking.
This is the nasty guy talking now. Some manufactures make it low purposely for a reason, so only their preamps that are dc coupled or have large coupling caps and low output impedances work properly with this sort of amp, and many others don’t, especially many tube ones because they are nearly all capacitor coupled and or high impedance, this gives them a sales advantage for selling their own preamp with that poweramp when an A/B is done.
Cheers George