Work that guy down as much as you can. Maybe snatch 'em for 90$ lol. But yeah, they should be noticeably louder, even sealed just due to moving more air. (surface area)
Then, if you still find yourself wanting the extra ~3db, build yourself a to spec, ported box.
Also, since you are on a stock H/U, this could be a reason for the sub to get quieter as you turn up the volume. Some have a feature that boosts low end on lower volumes and reduces it at higher volumes to protect stock speakers. Just food for thought if the problem persists with the CVR's.
Best of luck, pm me if you have any questions.
Edit: Don't let him raise the price on you. And make sure they work b4 purchase.