I have been using my Spider II 150 watt combo for years and have been completely happy with the sound from it. Recently, I picked up a Spider III HD 75 with a Spider 4 x 12 cabinet. I also recently picked up a Spider IV HD 150 head. With either the Spider II 150 combo, the Spider III HD 75 or the Spider IV HD 150 connected to the cabinet, I get tremendous bass response that I absolutely love. But now, when I play through my Spider II 150 combo by itself, the bass seems flabby and not nearly as tight. I am thinking the difference is because the 4 x 12 cabinet is sealed whereas the combo is open-back. I was wondering if anyone had ever modded one of these combos to enclose or partially enclose the back of the combo, and if so what were the results?