Well what did you buy it for? Use with xbox with the xbpx mixer? If so then they are great. If your using them for pc, aslong as the have audio jack cables they work fine as with mobile aswell.
I too have astros a40's. Got them last year brand new, not sure which gen, the latest i assume (note they are not the TR edition) and from one user to a another, these are great. Sound is amazing, comfort is beyond belief and the mic is good ( not exactly studio worthy but its pretty clear in my eyes) i use mine with my phone, my tv, my computer, and consoles (via tv audio jack) and works fine all around
NB i too do not have the mix amp, its waste of money and space. Costs as much as the headset and doesnt really do much, your pc can do all the stuff that the mix amp can for free