The listening area is a fairly large room 28' X 25'. The celing are fairly tall I think 13'.
Well, that's over 9000 cubic feet - a REALLY BIG SPACE! If you head over to the subwoofer sub forum and tell them that number, you'll get sub recommendations based off of it. For example: I have approximately 3600 cubic feet and I use dual PSA XS15s and I believe this is the bare minimum I would except for my space...I'm even thinking of adding another.
Is your space open to other areas of the house? If so, your cubic footage is even greater than 9000.
IMO, you're just not going to get good bass in a space that size without spending >$1K on a sub or (preferably) multiple subs....sorry.
Would you recomend that I try Biamping the fronts with the highs from my onkyo receiver and the lows from my dedicated 2 channel amp Or would that be a waste of time.
A waste of time.
The sub I am possible looking at if my wife doesn't kill me for wanting to spend another $300 on the audio system is the Klipsch rw12d.
The RW-12D is widely considered a great entry-level sub. But in a space your size, it's probably not going to accomplish much unless you have it placed "near-field" (i.e. right next to your main listening position) and even then I wouldn't expect miracles.
with that being said I am a newbie this is my very first ever quality audio system. So I do not really know what good is supposed to sound like.
Do you have any audio stores in your area where you could go and listen to some higher quality subs? Would give you an idea of what's possible.
When I turn the sub off the RTi12's have very little bass coming from them.
This is because your bass management is turned on and everything below 80hz is being sent to the sub. When you turn off the sub, all of those lower frequencies are lost. In order to hear what your mains are capable in the bass department, you have to do what I suggested earlier:
If you want to hear what your mains sound like with no bass management at all, set your mains to "Full Band", subwoofer to "NO" and Double Bass "OFF".
when I get to listen to pop music or electronic music in which bass is important, and it sounds like I am getting all my bass from the sub
This is exactly how it's supposed to work.
Those tracks probably have more low frequency info than the others you listened to, making your sub more active - which probably doesn't sound all that great because you're trying to fill a GIANT space with a very under-powered sub.