Have you checked the 414 to make sure you don't have the -12 or -18 db pad engaged?

It is set on 0.

What specific input are you using on the interface? Quote:"...balanced cable goes to an analog input of the Fireface."
You may have a ground issue here. Also, the phantom on the Fireface is OFF, correct? It should not be on unless a mic is connected to it. If the SPL is connected to the XLR input of the Fireface, you are running a mic preamp into the XLR input on the Fireface, this will cause issues. Please be more specific, thanks!

Hello, thanks for the answer.
The Phantom of the Fireface is off. The cable I am using is connected through the XLR connector to the output of the preamp, and through the TRS connector to one of the analog inputs of the Fireface. I have tried several inputs (Neutrik combo on the front panel and TRS on the rear panel), with the same result. Please tell me if the issue is there.

Personally I guess it is a problem depending on one of the cables coming out from the recording room (that actually is a box) and on a ground issue.
I am saying that because I have tried also to connect the mic to the preamp right in the control room with an independent cable, without any disturb.
Then I've tried all the other cables coming out the recording room, and the problem does persists.
By the way, verifying the cables one by one I think I have disconnected the mic too early, maybe when the Phantom of the preamp was not completely deactivated, although switched off.
In this way I explain the fact that now from the mic I don't get any signal, although it looks powered by the phantom.
Sorry for the mess, but this is what's happening.
Any tip is welcome.

I'm trying to get a handle on whether we are having a bit of a language barrier here?

When you indicated that you are hearing a disturbing low-frequency noise, how would you describe this noise? You indicated you were taking the microphone from a central multi-input box, in the studio? Which is then routed to your RME. And you get this low-frequency " disturb ". But when you plug a microphone directly into your RME, bypassing that central input box, there isn't any problem? Or is there still a problem? If the problem goes away when you are connected directly and comes back when you are in that central input snake box, this would indicate that there are other grounds from that snake box, to other ground points that, that box is also plugged into such as an alternate mixer, XLR patchbay, other preamps, other return feeds such as headphones that feed into that box for people in the studio and stuff like that. This will induce a huge ground loop with your equipment. It may require using to either lift your XLR pin 1 connections (meaning to disconnect them from the connector completely). Or, you could also take the chance by simply and completely lifting all of your electrical AC mains grounds you have your recording equipment plugged into. This would include your computer, the preamp and any other external items such as headphone amplifiers and such. Your electrical ground then would be dependent upon that single common connection between the snake box and the central ground from a pathway up the microphone snake to the control room mixer, equipment racks and such. Though there will be that element of possible shock hazards. So one should have an AC voltmeter in order to verify that you have no life-threatening 240 V 50 Hz between your equipment and ground. And that will likely cease the low-frequency 50 Hz ground loop I believe you are describing. This is frequently the fudge factor one has to contend with when running portable equipment in an environment, not completely of your own wiring.

What was NASA thinking when they heard a BUZZ on the moon?
Mx. Remy Ann David