It is a fact of life in the musical equipment industry that one manufacturer's 500-watt rating may be completely different from another's. Think of it as a variation of "Your Mileage May Vary" in the auto industry.
Here's the deal: It takes more power—a lot more power—to drive low frequencies than it does to drive high frequencies. When the amplifier specs are calculated, a commonly used test frequency is 1kHz (1,000 cycles per second). This means that the complete waveform travels 1,000 times in a second, producing a relatively high frequency. If an amplifier can deliver 500 watts at 1kHz, then advertising this amp as a 500-watt amp is not a false statement. The real question is about how many watts it does delivers at full bandwidth, or, in other words, all of the frequencies between 20 and 20,000 cycles (note: all SWR amplifiers deliver full 20 Hz to 20 kHz bandwidth). The fundamental frequency of the low "E" on a bass guitar is around 41.3 Hz. This is a large waveform, and it requires a substantial amount of power to amplify it at high volumes. When you ask an amplifier to reproduce lower frequencies, the amplifier becomes less efficient and therefore produces fewer watts. Realistically, an amplifier should be rated at several different frequencies in order to give an accurate representation of its power-producing capabilities.
An amplifier manufacturer will often put a filter on the amp that doesn't allow it to reproduce frequencies below a certain point. This typically makes the device more efficient and therefore capable of producing a higher wattage specification. In the case of a bass amplifier, if the filter was set at, for example, 20 cycles, then a 30-cycle (low "B" on a five-string bass) fundamental frequency could be reproduced. If the filter was set at 40 cycles, the low "B" fundamental could not be accurately reproduced, although the first harmonic at 60 cycles would sound OK. By placing a "high pass" filter on an amplifier, a manufacturer can publish higher wattage specifications that in reality are misleading because the frequencies the amp can reproduce fall short of the range of the intended purpose.
Another common way to bump up an amplifier's wattage specification is to show the wattage rating at a low impedance. The most common impedance that musical instrument amplifiers are designed to drive is probably 8 ohms (although a 4-ohm impedance is almost as common). Because an amplifier delivers a higher wattage output at lower impedances, it's not uncommon to see an amplifier advertised as 500 watts at 2 ohms. What does this mean at 4 ohms or 8 ohms? Less power!
Make sure that you learn more than what is painted on the front panel of an amp or is included in an advertisement if you're truly interested in the power-producing capabilities of an amplifier. One model's 500 watts may not be the same as another's 500 watts. Check the specs.