I agree with Walkinshaw in that i dont think the stacker will be compatible with ne other HU, try and sell the current gear, and use the $$$ for pioneer stacker.
Ditch the 6x9's for some $$, i just sold the same 6x9's only mine were 260w peak, for $150 to a mate, im sure u could make 200 if you pushed it.
Use the money for some cheap splits, that will improve the sound quality 10x compared to the 6" 3wy's. Then put the 6"s in the back so wont draw your front stage mids back as much as the 6x9's. Keeping a descent sound stage setup.
Hold it helps
1992 EB II Falcon
Alpine CDM - 7857RB source unit
Pioneer 6" 3-way front
Alpine MRD-M500 monoblock
2 Cerwin Vega 124 Subs
Descent Front Stage Splits
eg. Boston Rally's
Descent SQ front stage amp