The graphics show a low frequency modulation of the recorded audio. To find out where this originates you will have to examine the whole recording chain.

Bias or offset is a static thing, means not changing. You can not 'compensate' a low frequency modulation, only remove it by applying a DC filter or low cut filter.. This of course might rermove audible low freqeuncy components. You use which Fireface?

Not sure what happened to my answer, tried to post here.

1) Technically this is not a DC offset. What you see is a low frequency "hum".
2) Best is if you can remove that hum. If not possibly you will need to remove it with a high-pass filter.
3) Most probable cause from my experience is 50 or 60 Hertz mains interference created by a ground loop. Do try to assess what frequence the hum has.

To remove ground loops can be frustrating. You need to consider each and every part in the chain from mics to computer to see how and when it occurs. Typically be either removing thing by thing or by disconnecting all cables and the connect one at a time.

My suggesting is to first assess whether it really is a ground loop.
1) connect one mic directly to the FF800 mic input using your XLR-XLR cable.
2) Make sure neither the metal outside of the mic or the metal outside of the connector on the cable touches anything metallic
3) If you record using a laptop, disconnect the power supply to the laptop and run a recording using only battery

If the "hum" disappears here, you can start to connect one thing at a time to see when the problem reappears.

Most probable cause in my experience is
a) laptop power supplies
b) the connection between preamp and FF800 since you have two cables (can create ground loop) as well as possibly both connected to a grounded mains output.

In b) the solution is to modify the cables between mic preamp and FF800. A short descriptiion is to disconnect shell from pin 1 and disconnect shield from both pin 1 and shell on one side of the cable.

// Gunnar