Dear winstonia83, I did sound for a few years in church before, I too owe a GnL Bass, played through 1X15"s before(though I prefer 12"s), I also have a head w/o cab(LOL)and I have the following advice to spare.
Your problem appears to an acoustic problem or rather the lack of a suporting sub-woofers in your church. A thousand seater? That's Huge!(God bless your church for multiplying)Will the new setup create more problems? As bass frequencies travel, they become muddier. Those people sitting at the back will experience a slight audible delay if they were to rely on a single bass combo/Head+cab blasting from the stage. Not accounting for the ppl who absorb the sound as those frequencies travel. Those at the front will hear the bass very loudly, those at the back, little.
Is the stage elevated? Be careful where the amp is facing, I can guarrantee if the amp is facing upwards(45% usually in kickbacks)the sound will reflect towards the ceiling(some churches have high ceilings) and bounce here and there resulting in a nice "reverb/echo" sound. A nightmare of course.
I know how exciting it is to buy new gear, but you must seriously consult the sound engineers in your church for better advice on how the new setup will affect the overall sound.
Note: OVERALL SOUND. Will the sound engineer's mix be affected by the new bass setup? He doesn't have control over the bass sound AT ALL.
Not trying to be negative here, but I hope you can see the whole picture here. you can buy the best setup in town, but if it doesn't fit the church, money is wasted.
Seriously, look into a budget into buying sub woofers. A long term solution.