The only guy that could give us a Tech lesson about this devices from an electrical engineer point of view is Steve R, so please wake up Steve!

The reason I went for the Apogee is because it will work with my Mac or my iPad.
I tried to understand the specs of the iRiffPort, but there is not much info about it on their web site.
As far as I can tell, the iRiffPort should work with any app.
What do you mean by distortion? What kind of settings do you use on the GK app?
I actually haven't used the Apogee with the GK app yet, as the app is quite limited. But the GK app is capable of giving the GK tone clean or dirty. Do you use the boost channel? Do you use the horn? Etc?
You can take a snapshot picture of your settings and post it here. Which bass do you use? And which headphones?
Remember the iRiffPort will not manage to drive a "heavy" headphones!