I purchased a GK MB500 head a few months ago to replace the amp on top of my old Ampeg B15T Portaflex cabinet. Something I have noticed; when driving the single Eminence Legend CB 158-8 ohm speaker I now have in the Portaflex cabinet, the amp sounds fine, but has less high end than when I have it driving the Eminence and any other 8 ohm speaker. I know the amp is producing more power into the 4 ohm load, but I haven't owned an amp that changes its "EQ" when driving a lower resistance load. As an experiment, I got a longer speaker cable and faced the extension speaker away from me and about 10' away. I sat directly in front of the Portaflex, and played the amp with the extension speaker connected, then with it disconnected. The result; significantly more high end out of the Eminence in the cabinet when the amp is driving the 4 ohm load. The extension speaker is an old JBL E140 - 8 ohm in a homemade cabinet with no tweeter. Nothing special, but good reinforcement when I need the extra kick for a big room or outdoors. I even faced it into the floor one time to make sure it wasn't the source of the additional highs. Same results; the Eminence in the main cabinet sounds much brighter when the amp is loaded at 4 ohms. Are my ears playing tricks on me, or is it not so unusual for an amp to have a different frequency response when under a different load?