Zuma

Couple things. Let's say, for the sake of argument, you have to have the send/return enabled in both the settings and the strip in your soft mixer... I don't see why MAudio would do that but who knows. It could be that you just need them enabled in the settings dialogue but not necessarily in the mixer strip button itself.... at anyrate, you need to change the routing in your Pro Fire's router so that the strip you are using for your master bus output reads "mixer output". The way you have it set up now, you are returning the stereo output of the master bus back into Sonar and thus you are getting an inaccurate representation of the actual volume output you are exporting at... throw into that another -3.1 dB. You can't disable the returns in your settings because you are utilizing them, so it's locked. But try just turning off the button in your strip and make sure that strip in your router is set to "Mixer Output", otherwise you're sending the signal right back into X!, which is screwing up your perception of the true, overall volume output...
 
As pointed out already, leave your Master bus's fader set to 0 and adjust all the other faders you have feeding into the master bus, so that the Master bus doesn't overload and clip. your problem is one of improper routing within your sound cards mixer.

Zuma

Your problem is not the ProFire itself but the way you've got things routed within the PF's  mixer. That and you need to make sure your Master bus fader is set to 0db and then adjust all your other track/bus faders that feed into the master bus so that they don't overload the master bus and and cause clipping. You've already got everything recorded and routed to the master bus in Sonar, so you don't need any instruments routed to the first 8 strips on your sound cards mixer. set Sonar's Master bus output to one of those strips(1-8) and make sure it is set to "mixer output" and that you're not sending the signal back into Sonar.
 
 
See, if you're getting clipping to the point where you've got to lower your master bus fader then your levels going into the master bus are too hot. Capiche?  Basically what I'm saying is, your mix needs to be adjusted properly for volume and pan before the master bus. and then secondly you need to get your routing within the PF's mixer setup properly.

Zuma

You can't have the return enabled in the strip you're outputting the master bus to. Click that button off in the one you have it routed to now. and it's not that you can't or shouldn't pull down the master bus fader, you can pull it down to gain headroom for mastering, but not til you have the levels feeding the bus where they need to be.
 
I always feed the master bus as hot as I can get it without clipping, then I'll pull the master bus fader down -3 dB or so to gain headroom for any mastering I do. As for your sound card mixer, i'm not sure exactly how MAudio handles things, but you certainly don't want to return the signal you're outputting from the master bus back into the host. I just told you to route the master bus output to one of your other strips because you don't have the return button enabled on those. The one you've got it routed to now is return enabled... that's what I think is screwing things up, but again, I'm unfamilar with MAudio's soft mixer so I can't relate to it the same way I relate to EMU's patchmix. I would suggest reading the PDF manual for your ProFire til you've got a better understanding of it's routing configurations.
 
 
As an example, in my EMU mixer, if I insert a send/return in the mixer strip it will send and return the signal through a physical input/output. In theory it should be the same for you in the PF's mixer. So, you're either routing the audio back into X1 through a send, or you're routing to a physical output/ input on the ProFire via a return/send . You need to get to the bottom of which one you're doing... that is why you shouldn't have the software return enabled.