Forum Advice, Sub, Center and Rears

Hello all, I have been Lurking for a few weeks looking for info and reading many reviews. I am a total rookie at this but appreciate Quality Sound and Video.

I have recently purchased a HK AVR745 to run my HT/Music for the house.

My current setup is:

TV: Mitsubishi 1080P DLP
Front's: Klipsch KG5.2
Rear's: Speakercraft AIM8 four's
(Plan to use these as the sides after I buy rears for 7.1 setup)
Center: Mission 78C
Sub: None in place yet

I am looking to replace the center speaker, Purchase a new Sub (or two) and add some new rears. I have been considering a two sub setup as my new Receiver has two sub Outputs and I have a large open room, 30 x 21, this does not include the Kitchen area that is attached to one side.

I know there are many items here but I am open to any and all suggestions and hope to hear your thoughts.

P.S. I am also running a Speakercraft bb1235 12 channel Amp for sound in all rooms of the house/backyard

65% HT and 35% Music. I definatley like it loud!. I have been in a few of the local BB, CC stores and have been unimpressed with anything I have heard. They really only seem to carry the lower end of the spectrum.

That being said I am looking to spend about $750-$900 in total, Maybe that means two less expensive or smaller subs or one larger one for the room.

I have the ability to spend more if needed but am not sure I have the experience to appreciate the difference. I have no foundation for the Budget Number. It just seems like a decent entry point.

Thanks for the help Mojo. If anyone else can add anything to this I would be grateful for the opinion.