Stereo amplifier for a pair of passive subwoofers.

The trouble with inexpensive amplifiers for subwoofers is that they tend to be deficient in exactly places where you need them to be strong: power rating and/or power supply. Big transformers and big capacitors are expensive so that's usually where the cost cutting takes place for inexpensive amps--high power rating don't help much with puny power supply.

You may want to consider finding an inexpensive class-D switching amps using 250-watt or 500-watt ICE modules or similar modules. They don't require large power supply and the inexpensive one don't sound good in the mid and high frequencies but you don't care about that. The bass will be the best that you can get out of solid-state amps.

Unfortunately, I am not familiar with the many small manufacturers that make these amps inexpensively. But I think it is well worth your time to look for them.

Sogood51, transient power peaks is also a very important parameter.

Yes, most of the time amplifiers just coast along around 5-10 watts. But even within that period, there are transient signals (in milliseconds, too fast to register on power meters) which can consume huge amount of power in short bursts. That's what the extra dBs of headroom is for.

Typically, most good power subwoofers use amplifiers in the 250-500 watts range. Some, like the new Velodynes DD use around 1,300 watts (Class D amps). If space, weight, heat dissipation (for class A/AB amps) and cost weren't a problem, subwoofer manufacturers would probably use even higher power amps.