So if you were thinking of replacing your Mackie with the TT as your main recording interface I wouldn't do it. As a preamp in front of it then sure but I don't know that the amp models and effects are really worth the extra complexity. You still need an iOS device to adjust the tones so now its three devices, the TT, the Mackie, and an iPad. I wish Line 6 would develop a desktop module like Amplitube.
Yes, a couple of ways. You can run audio out from the mains out to your Mackie. You can also run the USB out directly but I have been having issues with latency using this method. The USB connection basically makes the TT your recording interface instead of the Mackie. So to monitor you either need to use headphones or send the audio to some kind of powered monitor. I found running the audio out from the mains to be the simplest and it didn't give me any latency or tracking alignment issues the way the USB out did. Interestingly the USB out method worked fine with Garageband but not Logic.