It sounds like a viable project. All the bits have certainly been done separately, but they may not have been combined.
The AMP2 doesn't seem the ideal headphone amp to me. There are other DACs for the Pi that have built in headphone amps and volume control. The iqAudio ones use the TPA6133A so they'll have >=10R output impedance.
For a bit more DIY you could try the OAP1622EVM - see TI's datasheets for measured performance. There's a thread or two at diyaudio.com about how to hook them up properly if you're not using a dac with balanced outputs. You'll need a power supply for that too, and look a the datasheet for how to connect the enable pin to avoid pops on power on/off. I'd expect that to be somewhere between the O2 and Atom. Or just use the guts of a DX3Pro.
moOde looks like it should cover your software desires - it supports most of the DACs for the Pi, generic USB DACs, eq, crossfade and Airplay. I don't think any of the other usual suspects (Rune, Volumio, PiCorePlayer etc.) have the crossfeed included.
Alternatively you can do the eq and crossfeed manually, but it may be tricky to get the setup right depending on what else you're using. The usual way is to add virtual devices to the alsa configuration for the bs2b crossfeed plugin and one of the equaliser plugins, and virtual mixer configuration for their controls. You then need to point the player software or pulseaudio to the right sound device. I've done similar things, but not tried those plugins myself. See
here for some old instructions that should still be mostly valid, but with different sound devices as it was done on a PC, and
here for a discussion of the same idea applied to Rune running on a Pi. I'm sure I've seen something on using EQ with PiCorePlayer but i can't find the thread.
