I read the specs on the Crate BT220 amp and it says it has an external speaker jack. Anyone familiar with this amp? I'm wondering if you hook up an external cabinet, will the 15" in the combo still be in use or will it shut down and just use the external cab? I know the BT50 would bypass it's internal 12" if you jacked an external cab to it. I want to have a combo with the option of using an external cab. With the Crate BT220 I'd like to be able to run a 2x10 with the 15" internal speaker if that's what the external speaker jack is for. So far the only combo amp I've read about that you can use an external cab with is the GK Backline 210 Combo. Thanks.