If you're not opposed to buying used, see if you can find a pair of vandersteen Quatro Woods. I got mine as a demo pair for under 10k. They're versatile and have built in room tunable subwoofers in each speaker which will be a huge asset to you with a room that size and limited treatments. Because the subs are powered the rest of the speaker is fairly easy to drive. Being time and phase correct they stage and image beautifully when properly set up and Richard Vandersteen always recommends rule of 3rd's to start with as you mentioned. If you don't want to go used. You can get a pair of Treo's for pretty close to that price and possibly add a sub later.
speaker choice for new 20x30 audio room
Finally getting an opportunity to set up a dedicated audio room. Its a separate out building from the house. 2x6 construction, fully insulated 20' wide 30' with 10' ceilings. Full HVAC and adding padded and carpet floor over 6" existing concrete floor. After heavy texture and new paint, I plan to add acoustic treatments, just he basics for starters, bass traps, wall panels, ceiling clouds and going with a rule of thirds layout for speaker placement.
For now I will be running everything from an Emotiva Pre-Amp, Peachtree 500 Amp, Bluesound Node for streamer and a Pontus II DAC.
Budget for the new speakers will be $5000 to $10,000. this will be a dedicated listening room, We listen to everything from Dianna Krall to Metallica Classic Rock, Blues, Country.. you name it. Just looking for recommendations of speakers I should be considering for this specific room dimension.