It's likely that the power supply failed. When the power supply FETs fail, they often make a mess. At the time when they fail, there may be visible flames for a second or two but as soon as the FETs open, the flames are gone. It may take a couple of minutes before it stops smoking (unless the fan was in operation at the time).
It's unlikely that dust had anything to do with the amplifier failing. Even if there is no air flow, the amp should have simply shut down (all amps have thermal protection).
If you've blown 2 amps and you're running it into a safe ohm load but you were running the amp hard when it failed, you may be asking for more than the amp can deliver. If the amplifier was not being run hard when it failed, there may be some underlying problem (intermittantly shorted transformer, defective insulator, broken leg on FET, shorted speaker wire...). These can result from poor installation, poor amplifier design or simply bad luck.
Before you install another amplifier, you need to check all of your wires for shorts. Use an ohm meter to be sure that each individual voice coil is OK. They are likely to read ~10-15% less than the rated impedance when using an ohm meter. If all of the speakers are identical, the reading should be very close on each. If you get one that's significantly higher or lower than the others, that speaker needs to be replaced.