Just got the Five Fifteen, and I really like the combo for what it is and the sound I get.
The External Speaker jack will defeat the internal speaker. So if you picked up the 2x10" Ampeg cabinet (or one of the Ashdowns) and plugged it in, it would get the 100w from the internal amp, and nothing would come out of the internal 15".
If you like the Ashdown, go out and pickup another Five Fifteen, get an ABY box and you can run your bass into both simultaneously for a 200w 2x15 stack. Right now you could probably get everything you need from Musicians friend for about $300 ($249 for the old cosmetic 5:15 model, $40 for a decent ABY). Then look to get a decent DI box to send your bass for FOH support.
Hartke Club #311, Greenboy Club #102
Spector Legend Classic 5, Warwick FNA Jazzman, Hartke LH500, fEARful 15/6 & 12/6 Cube, BFM Jack 210