Box Volume and Shape

Systems

What effect does the sub box shape have on volume and sound quality in general? Most dual sub set ups that ive seen have the wide boxes that are barely a foot deep. by comparison, some bass tubes that use similar sub and amp specs sort of sound better and louder, but of course i may be too subjective.

In any case, if there are some benefits, can i simulate a round tube by making a rectangular box? Since i don't want to mess with the stock mounting of my CD changer, im thinking of making a deep box, maybe 15" internal depth, and either install it between the changer and right footwell, or sideways behind the seats like a round tube.

I dont have a very large budget and since i dont compete in shows or anything im pretty much a DIY guy.

Thanks in advance for your comments and suggestions.