GIK sells good stuff (most "treatments" are crap IMO). Always start with a well proportioned, insulated, and dedicated room (most don't). Then find loudspeakers that match the room. Next read Floyd Toole's "Sound Reproduction" (either edition) to learn the basics.
I have six GIK 2ft x 4ft 244 "full range" panels that I've placed at first reflection points on front, side, and rear walls along with four GIK 2ft x 4ft 244 "bass traps" panels straddling the front corners in my 8ft x 13ft x 21ft (Golden Rectangle) well insulated room. Currently have Buchardt A500SE's set up 4ft from the from front wall and 1.5ft from side walls with my chair in the middle of the room. Also have three tall bookcases on side walls acting as "casual" diffusers that I can adjust. Overall a good room for a very modest investment.