Speaker Placement Nirvana

My Spendor S8e speakers are on the short wall of a 15.5 x 18 room. The faces were about 4’ from the back wall, and about 5’ from the sides. I changed amps from a NuVista M3 to a Cary v12, and at 50W the Cary seemed like it was having trouble controlling the bass. It was a little boomy on some albums. After moving them around laterally, I decided to pull them further out from the back wall so the face is now about 5’ out. Wow. What difference that made. The boom is completely gone and everything sounds terrific. The change was NOT subtle. So my question is, what happened and why did the bass respond that way? I could see it if I’d had the speakers placed very close to the wall, but they were pretty far out to begin with. Any thoughts?