new enclosure or new subs

You have to consider the vehicle difference first. Everything is louder in a hatchback car or suv then in a truck. Includding your current setup.

Look at all the room you had for your subs to produce bass compared to now where they are firing into the bottom of your back seat.

Try a simple test of taking your subs and sitting them on top of your back seat and have them firing forward.

Then try them firing upwards sitting on your back seat. Should be a noticable sound difference.

It's been awhile since I've worked on a Chev ex-cab and varies by year but is there any way to have your subs firing forward under the seat?

What size subs do you have, and about what kind of air space do you have for each sub?

I'm guessing 1.6 total. Is the box those dimmensions exactly or does it have some dips and bends to fit snug in your truck? If so then 1.6 total for sure.

Even if you went ported for one it would sound better but not super great  1.6cuft for a ported 12" box is kinda small. Would be ok for a single 10".

Back a few years ago working in retail we had a lot of problems with getting great sound out of small boxes with a lot of good name brand subs because they weren't made for such small air space. We did have a lot of luck with 400watts or more(a pair, 200 per sub) on MTX Road Thunder series subs and some cheap brands that you would get for free when you bought an amp, or box, or whatever promotion they would run.(they didn't sound great in much else but loved small boxes)

Now there are a lot of comapnies making flat subs and subs designed for small enclosures and tight airspace. I would try a few places to see if they have any available that you can hear in a smaller sealed enclsoure. That may be the way to go.