Question:

Can I connect the output of my amplifier to a line level input of a power speaker/sub?

Answer:

Not directly, doing so would damage the input of the device the amplifier is feeding.  Some amplifier models have a loop through jack that allows you to send the line level input signal to another device, but amplifiers without this feature would need a special transformer that could take the high power output of the amp and step it down to line level.  This would compromise quality.  A better and possibly cheaper way would be to split the line level signal feeding the amp and send that signal to your power speaker.​