It is the ratio expressed in dB between the two voltage values and is calculated using an equation. For ease you can find calculators online.
I remain unsure about your concerns from a practical level. You are saying in absolute terms that an output of 0.6v is too low to be of use and that any output above 1.5V such as 2V is too high to be of use. If correct that effectively gives you a dynamic range of only approx 8dB ( the ratio in dB between the two voltages). That is impractical.
As I mentioned earlier I wonder what you mean by “clip”. When an input sensitivity is stated as e.g 1.5 V it does not mean that a voltage below that will not be sufficient nor that a voltage above that will clip. It means that the stated voltage will produce a total of distortion and noise at a predetermined level ( e.g. 0.01%). Clipping is not usually considered as being reached until that figure reaches 1%. So are you saying that your analogue equipment will be satisfactory at 1.5V ( e.g. THD+N = 0.01%) but with half a volt more input that THD+N figure leaps to 1%?
I wonder if you are simply reading the figures and thinking that they all have to match exactly as if the output voltage figure of Bartok is a sort of key that will only fit the lock of an input that has the same figure?