Did a quick google search, and the woofer being asked about is the Sonace P800D.
Originally Posted by InWall guy With a 6 1/2 inch woofer in the ceiling speaker will adding the sub give me much better bass response?
Do I have to buy a back enclosure or could I just add two extra 2x4s to create an enclosure in the wall?Is it worth the extra cost? I know this last question is a very subjective.
An 8" will be better, but what would be best is if you got the powered version. This way it takes the load off your receiever, and gives you more control over bass management. 150Hz is awfully high for a crossover, and the 6 1/2" drivers in the ceiling should easily be able to make it down to 80Hz, which is a much more ideal crossover setting. What you would do is tell your receiver that your ceiling speakers are "Small", and set the crossover to 80Hz. Then, simply feed the subwoofer the LFE signal from your receiver.
You would also want an external amp because if you do get the passive version, which is crossed over at 150Hz, you'll easily be able to tell that lower sounds such as deep male voices are coming from the subwoofer, not from the ceiling speakers. 80Hz is harder to localize.
As far as the enclosure is concerned, I would get their enclosure. Unless you can get the Thiele/Small parameters of the woofer, it would be difficult to DIY the correct size of enclosure. Putting the woofer into a proper enclosure will get you better output. Just putting in extra 2x4s won't work, because an enclosure needs to be braced and airtight, except for the port (if there is one).
Is it worth the cost? I didn't notice any prices on their page, so I can't comment. If it absolutely has to be inwall, I'm not familiar with competing inwall brands, so I can't offer alternatives. If you're willing to go with a box sub, then there are options. Your budget and size limits will be the biggest determining factor.