If you want to extend your frequency response above what the 15" drivers can produce then imo you should be looking for mid range drivers and crossovers not 10" woofers. One cheap option would be a bi-amped PA speaker. I bet Crate PA speakers are a dime-a-dozen on your local FB marketplace. Here's how I would set it up:
The impedance issues brought up previously still need to be taken into consideration.
- Bass
- Pedal board
- Peavy Headliner 1000 front input (I assume you're talking about this amp but I'm not 100% sure)
- Effects loop out
- Crossover
- Low output goes to Peavy Headliner 1000 effect return
- 15" cabinet
- High output goes to secondary amp (can be lower power like Peavey PV260)
- PA speaker
- Low output goes to Peavy Headliner 1000 effect return
The impedance issues brought up previously still need to be taken into consideration.