Question about sound below an amps frequency specs

Most amps roll off gradually below 20Hz and above 20kHz, so unless the amp employs steep filters to purposely roll off the response then you should still have some output at 10Hz.

That said, I don't believe many humans can actually hear bass tones below 20Hz. Such low notes are felt more than heard. If the output from the speakers is high enough, objects in your room will start shaking, and walls, doors, and windows may vibrate loudly.

Most speakers will not produce significant output below 20Hz, and any bass you actually heard was probably an upper harmonic distortion (and not the fundamental test tone) -- especially if you heard bass while playing a 10Hz test tone and the room was not shaking...

In order to eliminate LF "noise" such as air conditioning rumble on the recording, or TT rumble when playing a LP, many electronic units do have a sharp filter below 20 Hz. The idea is that this signal wastes amplifier power, and drives loudspeaker cones in and out, which you can't hear but which causes distortion of higher frequencies that you can hear.

In general I think this is a good idea. However, I have one CD of a Wurlitzer theatre organ that has a continuous non-musical LF rumble at and below 20 Hz (per my spectrum analyser). I have heard the actual organ, which is in a pizza restaurant in Mesa, Arizona, and this sound comes from the wind generation machinery. If you filtered it out you might like the sound better but it wouldn't be like the real thing.

Most solid state amps will "pass" frequencies below 20Hz, but most speakers will not reproduce such low notes. My guess is that the tube amp sent 2nd and or 3rd harmonics to the speakers, and that is what you heard.

Will it pass info outside of those ranges?

Yes. How much........figure that each one represents a first order filter in the amp.

IOW........those numbers are 3 dB down, and one octave away would be 6 dB down. Two octaves would be 12 dB down.

Playing low frequency tones usually results in tons of distortion, and that is very likely what you heard.

Unless there is a reason to limit the bandwidth of the amplifer, the low frequencies will not be intentionally filtered just outside the published specs.

The frequency response of an amplifier is specified with a dB range. Your amp may be 20-20k +/- 1 db (or whatever). Any frequencies over or below simply will exceed the 1 db range.