Soirry for the late reply. This topic needs some solid documentation and we will provide it. The PEQ is proportional (or variable) Q, where Q is defined at 6 dB gain. The amount of variable Q seems to match very well with other popular EQs, for example the Oratory settings give the exact curve published. But there seems to be no definition and no further explanation or even mentioning of the amount of variability with Q, so this needs some more investigation.

It's more simple than I thought, just got confused by the Q definition. We indeed use the most popular Variable-Q EQ, based on the EQ cookbook recipe of Robert Bristow-Johnson, as many others do as well, so will be compatible to most.

What confused me first was the definition of Q as '50% gain' in combination with 6 dB (50% is -6 dB...). The meaning here is that the bandwidth is measured at half of that value. So if you set the EQ to +6 dB, you would measure bandwidth at +3 dB points (only here it matches the classic way of measuring bandwidth aka -3 dB points). With +9 dB gain at +4.5 dB, and with +12 dB at +6 dB. Etc.

The popular RBJ EQ works much more 'musical' and audibly better than the classic, gyrator based EQs that were very popular due to their low price. Those have a very wide bandwidth at lower gains, influencing a far too broad range. The opposite could be said from so called constant Q EQs, which influence a much too narrow area when using gains below +6 dB.

The RBJ version reminds me of an older Electro Voice EQ mode called 'Constant Range' which also worked in-between classic variable Q and constant Q, by keeping the influenced area independent from the gain setting. They managed to do this with clever analog circuitry, but the name and the EQ vanished, and even Google will not easily find it.

Back to RME - our PEQ looks like this:

Note that the Q values in that pic were taken at -3 dB points, the 'classic' way, which results in a higher value with higher gain. Using the RBJ way to determine bandwidth Q will be 1 for any gain.