Question about sound below an amps frequency specs

if an amps frequency response are spec'd out to be between 20hz to 20khz, will it pass information above and below those numbers?For instance will it pass a 15hz tone or will any information below 20 hz be cut off by some internal filters. If an A/v receiver has the same frequency specs in their manual, will it also cut off info above or below its stated specs? Could info going through its LFE be cut off below 20hz as well. The reason I ask is when I was playing a Realtraps bass test CD I downloaded, my speakers had no info below 20hz, though the disc starts with 10hz tones. Just for fun I played the same disc using my tube amp and had significant bass through my mains well below 20hz. Which made me wonder.