I suggest looking at the AudioKinesis Swarm Subwoofer System, which is not expensive for what you get. It is much easier to find speakers that sound great from about 50Hz and up but getting those lower octaves to sound right from your main speakers can really cost some money. There are benefits to multiple subs that you just can't get by using only your two main speakers. If you decide to upgrade your main speakers later, the Swarm will likely keep pace and do a great job filling in the lower octaves.
^^This. Or for super high value you could DIY your own DBA with one Dayton SA1000 and four 10" subs for about $2k leaving $3k for some nice small speakers, that will give you awesome imaging with awesome bass. The DBA helps with spouse approval because they are not fussy with placement and can go in corners or behind furniture.