[BScout]

You'll want low latency for recording (FPGA mixer loopback) to give cue mixes back.

However, you can set TotalMix to the simplified version (straight in to out). It's under Options> "Operational Mode" where you can change it from "Full Mode" to "Digital Audio Workstation Mode" which is straight routing.

Either way, TotalMix adds nothing to the latency/processing of the signal.

As for class compliant/Quartet.. class compliant is a bare minimum standard. It'll continue to work but why you would buy into something already abandoned is beyond me. There are already other class compliant audio interfaces that show the pitfalls of counting on that; they continue to work but parts of them that aren't part of the standard stop -- like digital volume control if that is operated via software or mic pre gain/trim. You'll end up with a "dumb" interface. Pretty much count on anything that requires proprietary drivers/software to stop working (so the stand alone stuff will work if it can even do things stand alone.) If you are going to go this route, get the still supported Apogee version and not the Avid version.

You should possibly also consider the HDn card and Avid Omni HD. It's pretty much designed for what you are looking for (surround output, 2 mic pre, low latency, simple headphone setup.) If you ever need HDX processing, you could always swap the HDn for an HDX card later.