|David Warren||Re: changing speaker impedance|
This will reduce the efficiency of the 4 ohm 15" speaker but I would solder a 4 ohm sandcast resistor, rated at 20 watts, or more, to the positive lead of the speaker. (Two 2 ohm ten watt resistors in series will accomplish the same thing.) The resulting impedance will be 8 ohms. This will in no way damage your amp. I am guessing that the 15" speaker is more efficient than the 12", so the resulting SPL and blend of the two speakers together may be balanced.