That seems like a lot of money to spend with little to know improvement. If anything you'd be better off just adding another subwoofer to the box. You could port it, but my guess at tuning is that you'd be in the low to mid 20's which would be fine for most people, but you'll get lower with it sealed. Can you fill the box with MDF scraps to cut the volume down? If you can get to the proper volume it will likely sound better. I agree it's time to stop throwing money at this problem. If reducing the volume works you can cut some MDF panels that you can fit through the driver hole and attach to the bottom top and sides.
According to the part express page 4.2 cubic feet
https://www.parts-express.com/Dayton-Audio-UM18-22-18-Ultimax-DVC-Subwoofer-2-ohms-Per-Co-295-518
I got some thermalight blocks at the yard they should do the trick shouldn't they?
Hard to say. It's kind of an odd material and I'm not terribly familiar with it, but I think its worth a shot. If it makes a difference, roll with it. If it doesn't, then I wouldn't rule out that the blocks were the problem. We're not really trying to absorb sound here, we're trying to remove volume.
You wrote "I'm looking for more low end extension without boosting on dsp", but reducing the box volume will do the opposite, low end (20Hz range) will reduce, and the 100Hz range will increase.
If the mains speakers are also playing in the same frequency range, the "boomy" 100 Hz region could easily be 10dB or more louder than the low end.
Simply reduce the level of the upper frequencies you find too loud in your room, or you will be making the problems worse.
We originally thought the box was too big but dayton site says 200litre is good for music. Maybe I have a bracing issue 🤔
Or room issues, or sub to mains integration issues, or expectation issues- post your measurements from several positions with and without mains and the issues would be more clear.
A sealed box will usually work well with lots of fiberglass, some of the common upholstery foam is not so good. Lots means to a point where you’re afraid you’re adding too much.
Flattens the peaks.
Flattens the peaks.