Sub In The Fireplace

Was wondering what your thoughts are on this. I currently have a Velodyne HGS 18 II, which I have tucked in my fireplace... The mains sit on either side, with their plane about 2 feet closer to the listener. The fireplace, which I obviously don't use, is pretty much in the middle of the wall. The sub fits easily with approximately 2 - 6 inches of breathing room on any given side. Being in a fireplace, the sub is surrounded by brick on 5 of its 6 sides. The house is on a slab. Is this a good thing? Am I losing anything having it there? Advantages, disadvantages? Should I consider insulating the breathing room the sub has all around it?
The location of the fireplace sounds good. Is it brick or pre-fab? If pre-fab you could get some really bad rattles--but you would have rejected the idea by now if that were the case. If it's brick--the brick isn't going to vibrate much--so any insulation is not needed unless the sub is rattling. The only concern might be the damper and the flue. I would consider closing it off and possibly putting in a heavy sheet of plywood layered with Owens Corning on either side. You could even go to the extent of using some small leveling jacks to push the sub down (put a sheet of plywood on it too) and push the plywood up into the damper. This would really make for a "sound" connection. The fireplace likely improves the efficiency of the sub--no loss of energy (or very very little) through it--so it's all directed into the room. If the fireplace is less than 1/4 wavelength of the crossover point (4.7 feet for 60 Hz) then I can't imagine it causing any strange anamolies in the frequency spectrum. If it's greater than 1/4 wavelength you might consider measuring it's response in and out of the fireplace just as a precaution. But it sounds like it's small.
I couldn't disagree more. I have a reasonably large masonry fireplace in between 2 main speakers, and the fireplace plus chimney gives a hollow boomy signature to the sound. The only solution was to wall the whole affair off with tube traps, thanks to which the speakers now have normal staging and a reasonably linear frequency balance. And that is with the speakers well out (55") from the front wall and fireplace.
Flex, difference is, his fireplace has a large sub in it and yours was trapping rear wave inside of a square hole. I'm not sure that the center of the rear wall is the best place for a sub but could be fine in some rooms, like Rives said, take a measurement.
Thanks for the responses... Rives, I guess I should have been more specific. But, yes, the fireplace is actual brick, and sits on the same foundation as the rest of the house which is a concrete slab. As far as the leveling jacks... are you suggesting this as a method to couple the sub to the concrete (in a sense)? Or simply as a way to cover the flue?

Sogood51, I wasn't sure if you were referring to me or not...
"I'm not sure that the center of the rear wall is the best place for a sub".

The sub is in a fireplace that is in the middle of a wall which one would face when listening to music.

And Flex, I see what you're saying... but I can't say that I've run into that same problem.
Sorry, should have read, center of the front wall. I need to start reading my posts over more often.
I measured my room response before, during and after treating the fireplace. Placing absorbers or diffusors inside the fireplace cavity, or covering the front with glass doors and partially treating the exterior did not work either. 8' tube traps do work.

Vectorman, if you haven't found the problem so far, then you probably don't need to worry about it.
By the way, I should mention that my main speakers at the time were rear-ported, which probably exacerbated the energy distribution into the fireplace behind the speakers. But Vectorman didn't mention what his main speakers are, and IME the speaker radiation pattern affects some room acoustic problems.
This is the best thread title I've seen in a long time! My suggestion would be to stuff the chimney with fiberfill and turn it into a giant transmission line by opening the flue... ;^)
The leveling jacks are to hold the plywood and Owens corning fiber board over the flue and while it is pressing up on this--it will also be pushing the sub down--just creating a very rigid positioning of the sub. It is very important to wall off the flue. Flex has shown the problems he had, but my guess is his fireplace was bigger and didn't adhere to the 1/4 wavelength rule for his cross-over point. This would most probably yeild a major bump at probably 2 low frequencies and a very colored disappation of energy. If you looked at a waterfall plot of a sub enclosed in this manner you would likely find at least one and perhaps 2 prominent frequencies that held energy for a long time. Again--this is why it's always good to take an acoustic measurement--then you can be sure. This is a theoretical model--and I have not tried nor measured the response in a fireplace (but now a really want to and compare those greater than the 1/4 wavelength rule and those less than the 1/4 wavelength rule). If you do measure it I would be very interested in your results.
Hi Flex, sorry I neglected to mention my speakers. My mains are an older pair of full range B&W DM3000. They are not ported. The sub, as I had mentioned, is a Velodyne HGS18 II, which is also not ported.

I like your idea of creative a very rigid positioning of the sub. I'm a bit unfamiliar with the wavelength rules your citing... e.g. 1/4 wavelength, 4.7' for 60 Hz, etc... would you mind explaining?
Sound travels at 1130 feet per sec. Thus (1130/60)= 1 wavelength for 60 Hz 1/4 of this. The reason I choose 1/4 wavelength is that it is impossible to have 2 peaked values.