New preamp, loss of low freq.?

If and I state if, this preamp has an output coupling cap this could be the problem as I’ve outlined above, it has nothing to do with the output impedance of your preamp which is nice and low at 50ohms being solid state, it's to do with the output coupling cap size if it has one.

As for poweramps that have 10kohm input impedance, yes this is very low and I really have a hate for this, not only does it create problems of preamps with to small a coupling cap as you "may" have.

It can also create this problem: 10kohm input impedance on power amps can also create problems with output impedance of passive or active tube preamps that have high output impedances, but this is another problem, and not associated with the preamp you have being solid state. 

Cheers George   

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