Heatsinks are great, but they have one problem: beyond a certain power level they use too much space, and, as Teemuk pointed, the fins have to be vertical for proper convection cooling, which also limits the size. Most of them are aluminum extrusions, and, obviously, the fins must be along the extrusion axis. You can get them as long as you want (I can buy them in up to 6 meter lengths directly from the factory) but the widh is limited to around 16 cm maximum. I have seen US made heatsinks up to 25 cm wide, they were VERY expensive. The hydraulic press and dies used must be monstrous. For me, the breaking ($$$$) point is 100/200 Watts RMS. For 50/60 W amps: just the 2mm aluminum backpanel; for 100W: 2 mm backpanel and ventilation slots in the cabinet or 115Hx50V mm heatsink with 20 18 mm vertical fins (small and very efficient), this one is *externally* attached to the aluminum backpanel . For 200W: two heatsinks as before. For 300W: a larger backpanel (500x200 mm), with 2 folded ribs reducing the apparent height to 160 mm AND a PC power supply fan (quite silent). It works far cooler (read: safer) than any of the passively cooled ones AND the transformer gets cooler too (very important in amps that work many hours non-stop). For 600/1000W: big horizontal heatsinks, dual-fan cooled (relatively noisy 220V units, 2/4 times more powerful than 12V PC fans). The fans pull air from the front or sides and blow hot air to the back. In theaters and stadiums, nobody hears them.