Since the Port Suggestion was done "in my head", I suspect the optimum will be around 7 3/4 inch each. Try the 9" depth I suggested and watch some of the punch come back. When the Punch sounds like it equals the extreme bottom, you are vey close.

Reactance VS resistance. Right now you have too much resistance, and you need reactance. The way the ports load and unload is what you are hearing. Sure is better to go too far than not far enough. I believe you will know when the sound "locks in".

Nice thing about port trimming, you can compensate for room problems as well.

Congradulations for having the courage to undertake this, and you will find it, and it will be awesome!

Glad you are having fun with it. Tweaking will now bring vast rewards.