Reply to Production Audio Interface

So i'm in the market for a Audio Interface for production.

I currently have an 2011 iMac i7 running through my Audio 8 interface going to my KRK VXT4's with a NI Kontrol X1 midi mapped to control volumes etc etc.

Iv been looking around and been recommended the RME Babyface or a Presonus interface and some others such as the MOTU. But i don't believe the Babyface would do me much good.

I have a budget of around 500 pounds for an interface and want it to be future proof as i will be adding a few items in the future such as the slim phatty, keyboards and stuff.

Also my other concern is volume control, as i don't ideally like having to use my X1 as a volume controller and i want to use my VXT4's for all sound from my iMac, So depending on the interface what other thing would you recommend such as the Big Knob?

Any input would be helpful.