Given your criteria, it sounds like AA might be a good choice for you. They allow upgrades using mileage on any international fare (including cheapies) and have MRTC (More Room Throughout Coach - 33-35" coach pitch) on all of their aircraft, both domestically and internationally. Plus the OneWorld alliance includes British Airways and AerLingus, and while you won't be able to upgrade flights on these carriers, you can use you AA miles to book award flights on them, if you find yourself going back to DUB again.

The downside is that there are only six daily departures out of TYS - 3 to ORD and 3 to DFW.