Help me with my 5.1 Surround Setup (pretty please)

Hi!

We're about to mix our film in 5.1 surround. I've done a bit of 5.1 mixing in Premiere and Audition and we tested it in a local theater and it sounded great but with only a 2.1 setup currently it was a lot of guessing and testing at the local theater. They were super nice to let me do that. But now I'm setting up my office to edit in 5.1 surround. I need to purchase speakers and the interface or controller to route all those speakers into and I'd love some advice. My office is 8' x 16' and has acoustic panels and 

The speakers I was looking at picking up were:

5 x JBL LSR305 MKII Speakers  ($99/each)

JBL 310s Sub

But what's a solid interface/controller option for less than $500 to run all those speakers into?

Focusrite 18i20 3rd Gen?

SPL SMC 2489? (I found one for a great price)

I'm not sure which of the above controllers is better. Does anyone here know?

Eventhough 90% of the time we'll just be editing stereo mixes for video,  I'd love to be set and ready for all 5.1 mixes as well. Thoughts?