It sounds to me as if you just have your inputs in Total Mix (including inputs 1 & 2) routed directly to your 1 & 2 outputs of the Fireface. You do this by raising the faders or via Matrix view. If I understand you correctly you're saying that you do not want to monitor your inputs via Total Mix, but instead you just want to monitor the output from Logic. If this is correct then you need to do a couple of things.

First open Total Mix and make sure that the Submix button is lit to make things simpler. When the button is illuminated the non-selected channels in the Total Mix gui will dim (darken). Select your 1 & 2 outputs (the lowest row of faders) and turn down the faders for the related 1 & 2 physical inputs 1 & 2 (above, in the top row of faders). While you're there turn up the faders on 1 & 2's middle row....the middle row is your software (DAW) output. With those top faders (physical inputs) down, you will not hear the device connected to your 1 & 2 inputs of the Fireface before it passes through Logic.

Second, by default Logic should be setup already to route it's audio output to stereo outputs 1 & 2, this is the audio that appears on row two in Total Mix. If this is setup correctly, you're done with Logic for the most part. You will now only be able to hear the device connected to your Fireface's 1 & 2 inputs if you have a track or channel in Logic set to these inputs and record enable it. If however you want to be able to hear the input without record enabling it simply turn on Software Monitoring in Logic...then you can just hit the yellowish icon/button that appears on tracks and channel strips. If you do not see this [i] button you just need to change what appears in your [i]track header, in Logic and add it.

Now you should only hear what comes out of Logic. :-)

Hope that helps.

Also, if that doesn't help or is hard to understand via my text RME does have a video on Total Mix. I've never watched it so I don't know how detailed it is, but it does exist. You can find that here. I can however vouch that the section in the manual is very detail and should teach you all you need to know about Total Mix.