If it sounds good it is good.  Why use a "more correct" setting if you loose the "fat" sound???  Adhering to rules and reference levels are a good place to start when learning, but breaking the rules to get the sounds you want is highly encouraged IMO...

To get a "correct" match, it looks like the RME's standard "+4" input setting would still net you +3dB of input headroom when the rackattack starts clipping.  But if the RME's "-10" setting sounds better, then the rackattack seems like its analog outputs might sound better at more conservative levels (well below the onset of clipping / possibly an output current limitation in the rackattack?)...

Set the RME to get the best level w/o clipping wherever you think the source unit sounds best (Otherwords, let the source unit's specific sound/volume relationship dictate where to set the RME's sensitivity to avoid clipping).  S/N will come into play at some point - but I'd be willing to sacrafice some noise floor for a better "fatter" sound...