I then performed some measurements comparing the electric output from Mic 1 and Mic 2. I connected the analog output from 5/6 to mic inputs 1 and 2 via a loopback cable. Note that I staggered the gain between both outputs so that it is easier to see the graph:

As you can see, both mic 1 and mic 2 look exactly the same!

So here is a summary of the results of my testing:

1. I have confirmed that the electric input between Mic 1 and Mic 2 are the same.
2. However, the measurements taken with a mic are different. One is low level, noisy, and the curves look different.
3. The mixer settings between the two mic inputs are exactly the same.

So why is the microphone measurement different between mic inputs 1 and 2? By process of exclusion, I have determined:

1. The RME Fireface UC is not at fault. Loopback measurements of mic inputs 1 and 2 are exactly the same.
2. The mic cable is not at fault. If it was, then the microphone measurements from the Mic 1 and Mic 2 input would look the same.
3. The microphone itself is not at fault. I spent money buying a new microphone just to prove that. And furthermore, the mic shouldn't behave differently when plugged into different outputs.
4. Therefore, the only variable is different Totalmix settings between Mic 1 and 2. But I copied and pasted settings from Mic 1 to Mic 2. There might be a hidden Totalmix setting I am not aware of.

Please check my logic and let me know if there is something I haven't thought of. I strongly suspect different software settings between the two inputs, but I don't know how to confirm that.

Guten Tag, Herr Daniel. Thank you for responding to my query.

Input gain level for Mic 1 and 2 with nothing connected and gain at 0:

Input gain level for Mic 1 and 2 with nothing connected and gain at 65:

The inputs as such look quite normal. Have you got a dynamic microphone to test this with?

Can you feed the same signal to the XLR input, using your Focusrite as a phantom power supply only, passing the signal through without gain? Just for a test? Potentially higher input noise levels aren't relevant here.

Yes, measure negative on 1 and you should read +48v on both 2 and 3.

I have done more experiments and I will summarize again what the issue is, and what diagnostic steps I have taken. 

The problem: I use an Earthworks M30 and RME Fireface UC. In the past, I have had no problem and it worked perfectly. Recently I noticed I have to turn the gain all the way up when taking a sine wave sweep, and I am unable to get an accurate reading below 80Hz and above 3kHz.

I own 3 microphones - an Earthworks M30, and two Behringer ECM8000's. I also own 3 interfaces - the RME, and a Focusrite 2i2 and a Presonus Audiobox USB. I need the 8 channel capability of the RME, and my only use for the other interfaces is for diagnostic purposes.

Experiments

Microphones. I have excluded the mics as the problem. All 3 of my mics work fine with the other two interfaces, although I am only able to sweep 2 channels at a time with those. If I use the RME, both the Behringers fail to register the sweep even with the gain set to maximum. If I use the Earthworks, I can get a reading but I have to increase the gain. I posted the faulty sweep with the Earthworks mic above.

Cables. I have excluded the cable as the problem. A check with a multimeter shows they are passing current correctly.

Interface: Through process of exclusion, the only remaining possibilities are a software issue, or a hardware problem with the Fireface UC.

The first thing I did was check the settings in Totalmix. Everything looked fine.

I ran a loopback measurement (DAC output --> mic input) with a cable. The graph is posted above. It looked fine.

I checked if it was outputting 48V Phantom Power with a multimeter. It registered 50.1V, so I have confirmed that the mic is getting phantom power.

I can not think of anything else to test. I am convinced it is either a hardware issue with the Fireface UC or a software issue, yet the limited number of tests I can do on the hardware reveals no faults, and looking at Totalmix it looks OK. Buying a replacement for the Fireface UC is out of the question at the moment because it is too expensive.

Please advise me on anything I have missed, or any additional diagnostic steps you might suggest.

I can try to borrow a dynamic mic*, however I do not follow the reasoning. How will a dynamic mic demonstrate the problem over my condenser mic, apart from removing the requirement for phantom power? Can you please explain?

* I just checked the price of dynamic mics and they are quite cheap, so I could buy one. Do you think a Behringer SL84C is good enough for this purpose?

I see. I have already run a loopback cable from DAC output to mic input (graph posted above). Would this show that the preamps are working, or am I misunderstanding the whole situation?

Sorry for all these silly questions, these things are quite foreign to me. Thank you all for your patience.

I see. I am in the process of trying to borrow a dynamic mic. If I can't get one, I will have to wait until the easter long weekend is over before the shops reopen and I can buy one.

I will try a listening test with those condenser mics which I own and see if I can hear a difference.