I was really frustrated when I started using summing mixers because I couldn't use any parallel processing without getting phase issues caused by hardware latency. I wanted to share how I was able to work around this problem in Pro Tools 11.
Warning: If you do not already know what parallel processing and analog summing is, you will not enjoy reading this blog post. Also, please note that I'm using Pro Tools 11 and that I do not know if this is necessary in other versions of Pro Tools.
Let me explain. Here is a photo of what I was trying to accomplish: several tracks of drums which are being summed externally, but that I want to also process in one stereo bus.
I must admit that I do not know why this doesn't work. It totally should. Technically it's because the signal paths (analog outs and parallel bus) arrive to the speakers at different times. You might assume, like me, that Delay Compensation would be able to handle this task. For some reason beyond my knowledge, at least in Pro Tools 10 & 11, you and I are both wrong.
The solution that I use is to add one "layer" of busses before the analog outputs. Look at this photo to see what I mean.
Now, each audio track is sending to two busses at unity, post-fader: one exclusive bus that goes to directly to its corresponding output (I hide these while mixing), and one stereo parallel bus. The audio track itself is routed to a "dummy" output bus that goes nowhere (if you choose "No Output" you lose metering capability).
And that's it! Now you can get the benefit of hardware summing and you can squash those sources in parallel without phase problems.