Regulations are often not concrete in their wording -I get it. I work with standards regularly in my field within the energy sector.
Because of this, guidelines, addendums, or supplementary documents addressing specific questions about the regulation or standard are frequently provided. These resources aim to reduce ambiguity and prevent multiple interpretations.
The FTC regulation is no exception.
"Shall be obtainable" means exactly that. If an amplifier is picked up by anyone, and the manufacturer claims on the packaging that it complies with FTC regulations, it should be capable of being tested at any frequency between 20 Hz and 20 kHz while delivering the rated power and distortion figures.
This testing could be carried out by the commission itself (as is done in the EU with random inspections) or by any third-party tester.
However, this does NOT imply that manufacturers are required to test their amplifiers at every frequency in that range. The regulation doesn't specify that.
If a manufacturer is confident in their design, they might only test at 20 Hz and 20 kHz and rate the amplifier based on FTC guidelines.
This approach makes sense. It's rare for an amplifier to exhibit low distortion at 20 Hz, high distortion at 1 kHz, and low distortion again at 20 kHz.