Me with my first Ribbon (MXL R77) compared to Shure SM57

https://youtu.be/s67PQ8ZoBKk
 
Just playing. Does the mic sound right to you? I'm supposed to test it right away to make sure it wasn't damaged in shipping.
 
I'm really excited about it, the more I listen to it. I'm trying to make sure it's working properly. It turns out there's a lot of rattle and hum in my room. 
 
I want to give thanks to Tatiana at Marshall for helping me get the Mic.
 
And lastly, the mic came with white gloves. Do any of you use white gloves when handling Ribbons?

Sounds like you made out on that mic deal. 
 
The difference between the microphones is so drastic that I wonder if it's not explainable primarily by microphone position rather than differences between the microphones themselves. You can get those differences by simply moving the microphone an inch or two from side to side.
 
And where IS that 40KHz content coming from? It's not your amplifier.

https://youtu.be/qdfWWg1FOyQ
 
Bit, it looks like the issue is the Audient ASP 880. I'm guessing it doesn't have a low-pass filter for the 96S/Sec sample rate. Can't be heard by humans so I guess they don't care. Makes me want to check 88.1 and below to ensure a filter is applied. I'm starting to think the ASP 880 is junk.

Yes, if there was in fact no filtering you'd get aliasing. But all ADCs employ filtering, both analog and digital. The analog filter is not at "Nyquist", meaning half the sample rate, but actually much higher.
 
What happens is that the initial capture is done at a very high sample rate, at least 64 times higher than the target rate and possibly as much as 256x. That temporarily puts Nyquist way, way above audibility and allows use of a gentle 6dB/octave analog filter tuned to a frequency up in the megahertz region.
 
After that initial conversion it's then knocked down to the actual target frequency by simply skipping over every N samples. Sounds wacky, but it actually works. Of course, when you record at 88.1 or 96KHz or 192KHz you naturally are going to capture inaudible stuff. If that becomes a problem (it usually doesn't, but it can) you can just insert a LPF into your track.
 
The thing is, guitar amplifiers aren't capable of outputting more than maybe 12KHz anyway, and anything above that is noise. Furthermore, dynamic microphones (whether moving-coil or ribbon) don't give you much over 15KHz anyway. There is consequently no advantage to recording a guitar amplifier at high sample rates, although it's certainly OK to use a high sample rate for the purpose of comparing microphones, if only to take the interface out of the equation.

I doubt the noise is coming from the microphone, so it would almost have to be originating in the interface assuming there are no other components in the chain (e.g. a separate mic pre or compressor). My guess would be that it's in the analog portion of the interface rather than in the conversion, e.g. a noisy op-amp.

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.

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.