I have a BA115HP, without the tube, and am thinking of doing the same thing. I'd also like to use the 15" as an extension speaker. There is room to put a couple of Speakon connectors on one side of the chassis out the back.
I'm going to have 2 connectors, one for the amp out and one for the speaker input. I'm planning on using 4-pole connectors to ensure I don't connect another amp output into the amp output of the BA115. I'm going to use the #2 pole set for the output of the amp on one of the connectors and the #1 pole set for the speaker input. I'll also have to make a special cable, one end to the #1 pole and the other to the #2 pole. (My other cables are all using the #1 pole so the ampeg amp will be isolated unless I use the special cable.)
This will be a pretty flexible setup. I can use the amp section of the BA115 to drive another cabinet or use the speaker of the BA115 as an extension cabinet. The only thing that stinks is I'll have to connect the cable to both connectors if I want to play it as just a combo.