Spent the whole evening redoing my port and I think I finally nailed the tuning

Been through the same thing with my last box. I had a slot port I calculated for 32hz and it felt gutless above that, so I shortened it and it went the other direction, turned into a one-note thumper around 40. Turned out I never accounted for the port sharing a wall with the box itself, wich effectively lengthens it. Once I factored that in and rebuilt it, the difference was obvious on tracks I'd been listening to for years.

What helped me after the rebuild was running a slow sine sweep from my phone through the amp and just listening for where the subs unloaded. You can hear the tuning point pretty clearly when the output drops off. That told me my math was actually right instead of me just hoping it was. One thing I'd add about the PVC, keep an eye on how close the port end sits to the back wall. Mine was too tight at first and you coud hear chuffing on big drops. I ended up rounding over the inside edge of the tube with a router and that cleaned most of it up without redoing anything.

Also good call on penciling out the layout first. I skipped that on my first build and ended up with a port that was a nightmare to get carpet around. Center shared port like yours is how I'd do it next time too.

Funny how the tuning number gets all the attention when the port area matters just as much. I learned that the hard way with my first dual 12 build. Tuning was dead on where I wanted it, but at higher volume the port made this fluttery noise on big low notes. Turned out two subs pushing thru a single small round port was just too much air trying to squeeze through, and the port was choking them. Once I swapped to a bigger diameter port the same tuning frequency sounded completley diffrent, way cleaner and louder. Worth checking yours at real volume with the windows up and your ear near the port. If you hear any chuffing or flutter on sustained low bass, the length math being perfect won't save you. Some people flare the ends of the port to smooth the airflow, that helped mine a bit too before I went bigger. Also don't sleep on bracing around the port, mine used to buzz against the wood til I put a small spacer in there. Drove me crazy for weeks trying to figure out where the rattle was coming from.

One thing that snuck up on me on my last build was how much internal volume the port itself eats up. I ran the numbers off the gross box volume, then measured the tube and its wall thickness and realized my net volume was noticeably smaller than what the calculator assumed, which pushed the actual tuning lower than I was aiming for. Since you swapped in a longer piece of PVC than the old setup, it might be worth double checking that the port displacement is baked into your volume figure before you glue the top panel down. Did you factor that in when you worked out the length?

Nice when the math actually lines up with what you hear. I did the same thing last year after trusting some online calculator blind, ended up a full 4hz off from where I wanted. Now I always cut the PVC a couple inches long on purpose, test it, then trim a half inch at a time until it settles where I want. Cheaper than rebuilding a panel because you cut one piece too short.

On my first center port box I fought the carpet wrap around that tube for a whole evening. What saved me was a strip of felt weatherstripping tucked around the port opening before the carpet went on. The carpet edge had something to tuck into and my trim knife had a clean line to cut against. Took me three tries before someone showed me that trick and I felt kind of dumb for not thinking of it sooner. Also on sealing, I used standard caulk on the inside seams of one box and regretted it, the smell lingered for weeks with two subs pounding the air through it. Regular flexible seam sealer cured overnight and never gave me grief after that. Your carpet stage should go quick if the port stays centered like you've got it.