Headphone practice amp, cab sim etc.

[Raz78]

Hi guys, new member and first time poster. Im considering building this little box for practice. Attached file should explain the idea neatly, cab sim permanently into mix ch1, ch2 mono in (for my metronome headphone out), ch3+4 combined with a dual gang pot for stereo signal like ipad or phone and then mixer out into the headphone amp.  Though id best ask for advice before i go all yihaa, is it doable or are there any obvious no no´s. Thanks in advance.