If you already have external preamps, I would focus on the interface and mix in-the-box. Most project studios are using a
mixer for playback and monitoring only. Although I like A & H, I think its the wrong tool for the job you're describing.
Get the signals through quality preamps/DIs (unfortunately, lots of cheap, bad ones) and then into your DAW via a decent interface (lots to choose from). That will have the biggest impact on tonal quality. A good reference
point that I've learned over the years is that it cost about $400/
channel to
purchase a quality preamp that will give you significantly better tone than the onboard Focusrite pres on a ProTools LE rig or a
Mackie Onyx
mixer. You certainly can get a different tone that may work on certain instruments/voices in certain situations but not better. The usual problems is that if you use that pre on multiple signals, the "Wonk" created by the tube or plate starved format will
build up into ugliness on multiple channels. You can go cheaper on DIY kits if you're good with a soldering
iron and have time.
If you're trying to record an entire metal band live and multitrack, unless you've got an acoustically controlled space, it is not going to give you great results regardless.
If you're trying to make quick changes into your interface, then a simple patch bay would be the tool to use. (and a lot cheaper).
Come out of your DAW into your
mixer for multiple headphone mixes via subs,
monitor mixes,
etc...
Don't confuse
tracking with mixing, and while we're on the topic, don't confuse mixing with mastering. All 3 are important and should be done seperately.
My .02
Phil