This information you can find in the manual and by reading the product specification.

Then you will see that the FF400's balanced line outputs were switchable between different reference levels up to +19 dBu @0 dBFS.

The analog monitoring port on the HDSPe FX card is mainly being used as phones port (TRS, unbalanced), but can also be used as unbalanced line output. It has no switchable reference levels and has a lower max output level of +13 dBu @0 dBFS, which is +6 dB lower compared to ref level "High Gain" on FF400 which delivers up to +19 dBu @0dBFS.

See also: https://archiv.rme-audio.de/en/products … adi_fx.php
"Analog Monitoring Output
The HDSPe MADI FX provides an analog stereo output in reference quality with RME's proven low-latency converter technology. The output is ideally suited for high- and low-impedance headphones.
Thanks to the powerful monitoring features of TotalMix, any input channel, recorded tracks, or complex sub-mixes can be selected for listening. An advanced Cue function allows each of the 98 possible stereo sub-mixes to be monitored with just a single mouse click."

From manuals

HDSPe MADI FX:
Stereo Monitor Output (Phones)
• Signal to Noise ratio (SNR): 115 dB RMS unweighted, 118 dBA @ 44.1 kHz (unmuted)
• THD: < - 104 dB, < 0.00063 %
• THD+N: < -102 dB, < 0.0008 %
• Crosstalk: > 100 dB
• Frequency response @ 44.1 kHz, -0.5 dB: 5 Hz – 22 kHz
• Frequency response @ 96 kHz, -0.5 dB: 5 Hz – 45.9 kHz
• Frequency response @ 192 kHz, -1 dB: 5 Hz – 66.5 kHz
• Output: 6.3 mm / 1/4" TRS jack
• Output impedance: 75 Ohm
• Output level @ 0 dBFS: +13 dBu

FF400
DA, Line Out 1-6, rear
• Resolution: 24 bit
• Dynamic range (DR): 110 dB, 113 dBA @ 44.1 kHz (unmuted)
• Frequency response @ 44.1 kHz, -0.1 dB: 1 Hz – 20.4 kHz
• Frequency response @ 96 kHz, -0.5 dB: 1 Hz – 44.8 kHz
• Frequency response @ 192 kHz, -1 dB: 1 Hz - 80 kHz
• THD: -100 dB, < 0.001 %
• THD+N: -96 dB, < 0.0015 %
• Channel separation: > 110 dB
• Maximum output level: +19 dBu
• Output: 6.3 mm TRS jack, servo-balanced
• Output impedance: 75 Ohm
• Output level switchable Hi Gain, +4 dBu, -10 dBV
• Output level at 0 dBFS @ Hi Gain: +19 dBu
• Output level at 0 dBFS @ +4 dBu: +13 dBu
• Output level at 0 dBFS @ -10 dBV: +2 dBV