If you can't find a 220v version, the Behringer amps should work fine with a step-down transformer. The Bash only lists 60hz input, so it may not work. It also has a high pass filter set, although this can be changed substituting different resisters using this chart
. The Dayton SPA1000 is switchable for 220v operation, but may also have a fixed high-pass filter, -3dB@18Hz. I could not find it mentioned in the spec sheet, so I am not sure.
Doubling the power, say from 500w to 1000w, will get you less than 3dB of additional output. The driver will hit xmax with 950w at 20Hz, so if you have a more powerful amp it will give you more headroom, but not more output above this. For music only, you could set a higher high-pass filter and give the driver more power above 30Hz.
I don't think you will want to stop the fan in the amplifier, as it does not have sufficient heat sinks for fanless operation. There are instructions for fan swaps to put in quieter fans, but the fan may still be audible during quiet passages depending on the location of the amplifier relative to your listening position. I did see one person who applied VGA heatsinks to the chips, and cut a hole in the top of the case to mount a 120mm fan. This should allow for very quiet operation, but is a pretty extreme modification.
You will want the legs long enough to give more open area than the area of the woofer, about 182cm2. I would think 5cm is enough if on a hard floor, a bit more if on carpet. I would now worry too much about sealing the speaker and plate amp. These are designed to seal with the cabinet, so as long as the surface is smooth and flat it should not be a big issue. You just want to pay attention to the possibility of an air leak when assembling the components.
I don't think you should expect too much below 20hz from a single 18". WinISD shows about 105dB at 20Hz at xmax, which according to the Fletcher-Munson
curve is similar in audibility to a 60hz tone at 1kHz. You may find some items in your room vibrating at this level, and room gain may give you a a few dB at this frequency as well. SPL at higher frequencies will be more, unless you use EQ to flatten the response. DSP will give you more flexibility, but you would need a measurement microphone or SPL meter to make much use of it beyond setting a high-pass.