It is often said that a good rule of thumb for putting db volume changes in perspective is that a 10db increase subjectively sounds "twice as loud."
It is also important to keep in mind that the subjective perception of loudness is highly dependent on the dynamic range of the particular piece of music. A classical symphony, for instance, typically covers an enormous decibel range, from the soft passages (which typically comprise most of the music) to orchestral peaks. That will result in a given volume control setting producing a much lower subjective perception of loudness than the same setting would produce on highly compressed material (that maintains a fairly constant volume), such as many popular releases.
Re voltage increase vs. db increase, Kirkus is of course correct, but I'll add that the formula to convert the ratio of two voltages to db is 20*log(v1/v2), where v1 is one of the voltages, v2 is the other, the asterisk denotes multiplication, and "log" is logarithm (base-10). If you put the smaller number on top you will get a negative result; if you put the larger number on top you will get the same answer but with a positive sign.
To convert two power levels to db, the formula would be 10*log(P1/P2).
A given db change anywhere in the path through the system will produce the same resultant volume change, whether it is output voltage, input sensitivity, or speaker efficiency. Assuming, of course, that nothing is clipped or overdriven as a result.
Regards,
-- Al