So I'm set on getting the UX2 and the KRK Rokit 8, but have some Q's:

1. What is the difference besides the extra inputs between the GX, UX1, and UX2? (Only one guitar will be used). Does the UX2 come with extra software or something?

2. How do I connect the studio monitor to it? It has RCA, TRS and XLR outputs.
The software is the same for all but they might come with different amp packs...ud have to check their website...as for connections they have 2 1/4 inch trs outputs for left and right
The GX has only one instrument level input, no mic input or phantom power. For wiring up KRK's it only has one headphone out, so you'd need a splitter and a few adapters(far from ideal, I do this and its a pain in the arse)

The UX1 has one instrument level input and one XLR for mics (no phantom power I dont think so no condenser support basically). Two analogue outs that you'd wire up with two 1/4" jacks to the KRK 1/4 inputs.

The UX2 has two instrument level inputs and two XLR inputs with phantom power. The same two analogue outputs to wire up to KRK's with 1/4" jacks. UX2 also has spdif so you can connect two more channels to it later if necessary. UX2 comes with the fxjunkies tone pack for the Pod Farm plugin.