I use MixIR too, sometimes, though I don't have an analog guitar preamp/head so mostly I end up using S-Gear which comes with an IR loader and a few RedWirez IRs included.
MixIR has way more features to combine and pan them though, the only thing I never really understood was the way the panning works. I would think a hard pan would leave the opposite side mute but that's not the case. When I asked them about it they said it's by design and gave some explanation I can't repeat because I didn't get it.
That said there usually is no need to hard pan and it works pretty well when you use your ears.
I'm not enough of a pro guitar player to fully recommend the Big Box, guitar tone is so personal and some people swear by a certain brand, but I enjoy the amount of options it gives you - they sampled many many cabs with many different mics at various distances and angles. It's all very well organized into folders so it's easy to experiment with them.