I say no based on the following logic.
Electronic devices are designed to run on a very specific voltage. If they accept a range of voltages, then they usually will regulate the voltage to the voltage they need, especially if their output voltage and current does not vary with input much like your modem (and unlike stereo amplifiers which will output more voltage with a higher input voltage)
This being said, the electronic device AFTER the regulator would draw the same current and voltage (if the regulator output a perfect voltage) meaning it would draw the same power (watts) as well. The regulator circuit is probably not 100% efficient so it would actually waste energy in its conversion up or down. So in the end, you would draw more power if you input a voltage other than the designed operating voltage.
Since non-switching power supplies voltage varies with curernt and therefore have a large range of output voltage (12V PS might output 17V under no load conditions), it is likely your model has some sort of voltage regulating circuitry.
If there is no regulated voltage, then we can assume that the electronic components do a have a voltage range they can work in, BUT it usually is small and therefore you could only lower the voltage a very small amount.
This leads to two logical conclusion. (1) you can input a lower voltage and it will be regulated back up leading to HIGHER power draw or (2) the voltage is unregulated which means you don't have the option of lowering the voltage a significant amount to see any real gains.
That being said, efficiencies vary greatly between power supplies. Power supplies are usually rated for both output AND input. So it would be a better bet to find a power supply that draws less amperage from the wall given the same output current with a similar ripple voltage. Or search out a way to tell a more efficient PS from the less efficient one. That is a whole 'nother topic.