I just bought the Ashdown ABM 500RC EVO rack head and was just wondering, there´s just one speaker output and its at 4omhs, i want to get a 1x15" + 2x10", so i was wondering about that subout at front at the amp, would u plug the 1x15 in the subout? and is the subout at 4omhs too? or should i buy two 8omhs cab and plug them in the speaker out. so i was just wondering about if the subout was at 4omhs and it would be wiser to buy two 4 omhs cabs and plug the bottom in the subout??