I have a combo amp that is 120w thru a 15" speaker. I was looking in my Musicians Friend mag, and noticed they had replacement 15" speakers that were capable of 300w. My question is, can i add this to my amp, take out the current head from the combo, and add an external one? Or could i just add this, and keep the internal head that is included in the combo. I just noticed that this mite be a cheap alternative to a new amp. Thanks.