If the only thing in the P/A are vocals and keys why not pick up a decent rig for the keys and get them out of the P/A? If everyone is coming off the stand then thats the way you should work it. Why have one instrument in front of the backline, that makes no sence, if one instrument needs a little boost from the mains I can understand that. Theres inexpensive decent amps out there for keys!! I think those kb 300 peaveys are decent and a 15" with a good horn can cover it fairly well. I'm not much on peavey gear but I know that little rig works nicely.
The first thing that goes into your front house is always your rythmn section;drums-kick/snare/HH. /Bass DI or Mic/ then whatever is your main rythmn piece guitar/piano?
Thats how you start building your sound, and yes you do need subs underneath to be able to add strength to your sound production. Remember that your P/A is for "reinforcement" that means your backline sound is what you like and need to be able to simply move it forward to your mains. If the drummers drums sound like crap without micing them there gonna sound like crap miced too!!! Yes an EQ can help but you always start with quality and move that sound up and into your mains.
I used a complete Carvin system from 2001 thru 2008. Their 12 ch. powered mixer to run the tops and 2 moniters, and a 2,500 power amp for the 2/18s it worked very nicely. The cabinets were the PM 15 on top and their upper line 18s (can't think what the model was) underneath. The subs were decent but weren't the numbers they advertize as far as low end but they all lie about that. good luck, if you want a "balanced" sound you can't have one piece in the front and everyone else in the backline, if you sound is soft and your 15s are close to the backline you can do it like that but if your set up is traditional your balance is physically off. The piano will be 10 to 15 feet in front of everything else. Doc