Now, from the most subtle to the most profound...
Level 1- The improvement can be subtly, though still consistently, heard when switching to the superior component (A/B), but it is not heard when switching back (B/A).
Example- Ars Acoustica Prototype I.C. Cable Vs. Coincident Extreme I.C. Cable (between the phono stage and line stage)
Level 2- The improvement can be heard when both switching components, and when switching back, but it is no longer specifically heard after a very short period of time; sometimes seconds, but almost always less than one minute.
Example- Coincident "Kamikaze" Phono Stage Vs. Jadis JP-80 Phono Stage (midrange only, for around 30 seconds)
Level 3- The improvement can be heard at length, but mainly only when making an effort to listen specifically for it, so it is not "obvious". This improvement is usually not "significant"; meaning there's a good chance that an audiophile may be able to remove this improvement from their system and still not suffer from its absence.
Example- Bent Audio Silver SUT Vs. Coincident Statement SUT
Level 4- The improvement can be heard all the time, and without any effort, by an audiophile. However, it would not be unusual for it to be not heard by those listeners with no interest in sound quality. This improvement is still usually "significant"; meaning an audiophile will almost always suffer from its absence.
Example- Graham Phantom Supreme Tonearm Vs. Graham Phantom II Tonearm
Level 5- The improvement can be heard at all times by anyone with healthy hearing, including listeners with no interest in sound quality. The improvement is now always "significant"; meaning an audiophile can no longer enjoy their system without this specific improvement.
Example- Coincident Statement Phono Stage (Latest Model) Vs. Coincident Statement Phono Stage (Original Model)
Level 6- The improvement is "transformational"; meaning not only would it be completely unthinkable to live without it, but the improvement actually alters an audiophile's thinking and perspective on both their particular system and "Audio" in general.
Example- Reference Lenco L-75 Turntable/Graham Phantom (Supreme) Tonearm Vs. Forsell Air Reference Turntable/Tonearm