Great to find an AVR community here! Let's get some good content going. I used to work on some homebrew AVR-based projects (not Arduino, raw atmel ASM/winavr-gcc). I am reviving one project slowly...
Currently enjoying learning KiCAD, to move away from EagleCAD.
Honestly, I have just gotten back to it after a few years... the projects I am resuming were atmega8, but now I want to try the 88a, as it uses way less current and it looks like I can run at 4-8mhz on just MIDI bus power, yay no battery!
I will look into the families you mention, I am waaay out of date :)
For MIDI... I'm thinking AVR DB is the best of the new chips.
3x Rail to Rail op-amps, the signature of the AVR DB.
Multi-voltage I/O -- Port C runs on a second power supply, meaning you can mix 3.3V (main power, for portA, PortD, etc. etc.) and then 5V on PortC. Basically an integrated level-shifter.
Full 24MHz speed at 1.8V -- No more looking at charts to figure out what speed you can go. 24MHz is always available (true on all DA / DB / DD and EA chips).
10-bit DAC -- You'll probably need to use one OpAmp to make this DAC usable as an output, but hey, its an option.