Smelly Powder Room

I have been replacing all 4 toilets in my house with Drake II's. The first two were done about a year ago, no issues. The 3rd one is causing me fits.

My house was plumbed with the thick all-PVC flanges that sit just above the floor. This total height was too much for the Toto (it rocked during a dry fit test), so I replaced the flanges. I used the kind with the stainless ring, and ensured they were resting on the floor (rather than just above it as they had been). I used a standard wax ring.

No problem with any of this so far. I've done a bit of PVC work before, and have set a few toilets.

After this, the powder room smells of sewer. So I figure I messed up the wax ring.
-Pull the toilet.
-Ring looks fine. Centered on the flange, compressed evenly all around. Looks fine.
-Clean flange and underside of toilet (nothing there - all wax stuck to flange)
-Install new wax ring
-Install toilet

Same smell

-Repeat process. Again ring looked fine.
-Clean and repeat.

Same smell.

Ideas? My next step is to remove the toilet, plug the line and see if the smell goes away.

I see no signs of leaking from anywhere at anytime (I can see the plumbing from the basement) during all of this. Toilet works fine. Smells bad.