Problem isn't that the head is too powerful. Problem comes when you can't hear the 15 farting out because the 2x10 is louder and still comfortable, so you don't turn down and the 15 dies. Had it happen with that combo of speakers before. Always better to have matching cabs than mixed ones, so nothing is out of balance. Two different cabs have different impedance curves so at some frequencies, power won't be distributed evenly, and that frequency can be one where the speaker copes badly with the power.