Some info from the manual. What mode is recommended?
Band 1. Peak, Shelf, High Cut or High Pass (Low Cut)?
All filters are adjustable from 20 Hz to 20 kHz, at a Q of 0.5 to 9.9. Cut/Pass have a fixed 12 dB/oct filter steepness.
Band 5 Type Available settings are Peak, Shelf or High Cut. High Cut is adjustable from 200 Hz to 20 kHz, at a Q of 0.5 to 5.0 and a fixed 12 dB/oct steepness.
Band 1-5 Q Quality factor is adjustable from 0.5 to 9.9 in bands 1 to 3, and 0.5 to 5.0 in bands 4 and 5, in steps of 0.1 dB. This equals a bandwidth setting of 2.54 (0.5), 0.29 (5.0) and 0.
Also what is safe levels. 6 db increase/decrease max would be considered safe?
Peak filters are going to be used to increase/decrease a specific region. Say your headphones have a dip at 3khz, you would add a peak filter that boosts the 3khz range.
The Q factor; adjusts how wide/narrow the peak filter is applied. The higher the Q value, the narrower the peak, and the lower the wider. For example if you have a peak filter at 3khz, with a really low Q factor, it will extend the peak further towards ~1khz and ~5khz. If you use a really high Q factor it will keep the peak narrower on 3khz, extending only to ~2.5khz and 3.5khz.
A Shelf adds either a boost or cut to an entire region up to, or past, that point. So if I want to boost the entire bass frequency from 0hz to 200hz, I could add a shelf at 200hz with +2.0db, and it will raise everything up to 200hz by 2.0db.
The PEQ on the RME seems daunting at first but it's really quite manageable with the manual and a little tinkering.
I'd love to offer more assistance if you stumble setting up your desired EQ.
For your specific headphone needs, you'll want to boost any frequencies that has a big dip, and lower any frequencies with big spikes.