It depends. It could have discreet op-amps for each channel, or there could be one op-amp for each group of 4 channels (four amplifiers in one chip).
 
And all the amplifiers are going to share a common power supply, so if there was noise coming from there it would be distributed among all channels. This is a pretty common problem, and all it takes is one leaky capacitor to start making trouble.
 
Either way, it's likely an analog rather than a digital problem. And I wouldn't worry too much about it, as 40KHz is easy to remove without affecting sound quality. For starters, don't record at 88.1 or 96 KHz as that alone will assure no 40KHz content.

Lol. Yep. I've been really pretty happy with the device so far.
 
I don't mean to be a whiner. I did just have the huge win of working with Marshall on a Mic that was indeed a problem, so thanks for helping reassure me about my interface.

I'm a big fan of ribbon mic's on guitar, and was somewhat surprised at the extreme differences between your two mics.
 
Just curious, was the ribbon positioned in front of the cone and the dynamic nearer the edge?
 
I like to place the ribbon about a foot from the speaker and about 10 inches above the floor.  IMHO this gives a very honest reproduction of the amp.

Both mics were about 5 inches from a cone.
 
The amp was a fender 4x10 Deville.