Choosing the best amp for a given headphone is a complex and personal task, and there are a variety of factors that influence how a specific amp will make them sound. Though there is definitely a relationship between output power and volume, impedance and sensitivity play a big factor in that equation as well. The relationship between voltage, current, and resistance/impedance is defined by Ohm’s Law -- you may want to read this article for more information and some discussion. You may also enjoy reading our tech article on the subject of Sensitivity, Impedance and Amplifier Power, which goes into quite a bit of detail about power levels and how they relate to headphone specs, while keeping things as simple and fun as possible.
As long as the amp of choice has a low-ish output impedance (the rule of thumb is about 1/8 of the headphones' rated impedance or less), it should be able to drive our headphones with a modicum of power. Any good amp with an output of 50-500mW at the headphone's rated impedance (depending on the headphone model) should have no trouble driving them. Our webpage specs list the Recommended power level for each model, which we use as a starting point. In actual listening, we sometimes find that more power is better-- especially for our higher-impedance models like LCD-3, LCD-4, etc.
To find out how much power a given amp can deliver to a specific headphone, you can look at the amp's specs and/or ask the manufacturer. As long as you can find out either the Wattage at a specific impedance or the overall Voltage delivered, you can determine if the amp meets our guidelines for the Recommended power level for that model. There's a very handy Ohm's Law calculator here, which you can use to find out the Wattage delivered to a specific headphone if you know the Voltage it can deliver.
As an easy example, here's what the calculator above tells us an amp needs to deliver in order to reach the recommended levels for the 200 Ohm LCD-4:
By way of explanation of the above: since we know the headphones' impedance (AKA resistance) is 200 Ohms, and what we want to see for power is 0.5 Watts (500mW, as defined by the specs on the LCD-4 webpage), the calculator tells us the recommended Voltage output is 10V-- assuming low-ish output impedance, etc. So if you're looking at an amp that can only deliver 6V, you may find it to be a little underpowered for LCD-4s. Likewise, if the amp you've got your eye on can deliver 14V, that should be icing on the cake so to speak...
With all this tech-speak and gobbledygook said, the numbers here are more of a guideline than an actual rule-- as with all things in audio, the best way to determine if a combination works is to try it. We suggest experimenting to find what works best for you and your needs.