Hiya
I found myself in your position last year with an identcal piece of kit. I'd bought the EB180-15 combo to rehearse with in a small cellar, which was more than enough power on tap. I started to struggle when we started gigging competing with 2 guitarists, I was hitting lots of red lights on the VU !! I decided to add an extension speaker and got a Hartke VX210. TBH I didn't notice too much difference on stage.
I ended up buying a Hartke HA350 amp and used the 2x10 with this for smaller gigs and band rehearsals and have added another 1 X15 cab for max revs!!
What I'm saying is - think it through before you add an extension cab, it might be worth buying a bigger amp altogether if you're going down the gigging route because you're going to come to a stage sooner or later where the EB is out of its depth. I use mine for home practice these days and obviously its there as back-up if required.
C
PS my extension cab was 8ohms