I usually listen to tunes using my Samsung Note 3 phablet and the Android app PowerPro, and have recently bought myself some Bose QC15's as there are times when I need the ANC. Please don't bash the Bose as I bought them cause they were the only decent headphones with ANC I was able to try out in the store before buying-now that headphones are not returnable after opening the package.
I know a lot of audiophiles buy headphone amps so I am wanting to know the pros and cons of them. Also what is the primary reason most people buy them? Is it just to boost the volume due to the device they are using ie; ipod, phone, tablet etc.? Do any of the headphone amps come with a built in equalizer? There are times when the built in equalizer for the PowerPro app is inadequate and I'd like to boost either bass or treble, etc.
I recommend you look around some more, its not hard to find what an amp is for. In your case, I would start at your source and see if you're getting the best signal possible there FIRST. If not you would just be amplifying a bad signal. Remember 2 things, a chain is only as strong as its weakest link, and loudness does NOT mean better sound.
You'll prob. want to invest in a DAC first, or you may look into something like a Fiio E17 which is a DAC, amp, AND will give you some EQ options (which many portable amps do not).
An amplifier is basically an electronic device that increases (strengthens) the amplitude of audio signals which pass through it. Usually this means an increase in current or voltage or both to achieve that amplification.
Typically an amplifier is used if your output source is too weak to provide enough volume for harder to drive cans (ie with either higher impedance or lower efficiency).
They're also used to provide enough power to achieve a more linear response. EG the AKG K701 are a relatively low 62 ohms, and a sensitivity of 105 dB SPL/V - but are regarded as being harder to drive cans. Although you can get sufficient volume out of even an iPhone, the frequency response can be altered with low powered devices because there is not enough power to produce the linear frequency response they are capable of (the bass suffers). Add a decent amp - and you get the full sound.
Some amps do come with tone controls (eg vintage receivers) or switches (bass or treble boosters) - these are basically hardware EQ that essentially provide more volume to a set frequency range. This can be done with any decent software EQ instead.
As far as your QC15 goes - they're good headphones (the NC is world class). But they're designed to be used with portable lower powered devices (eg iPods or other DAPs), so adding an amplifier is not really going to help you. It'll just add cost and bulk for no real benefit.
Perhaps a better option would be to see what other apps are available for your Note 3 - and if they have better options for tweaking the sound. It should be cheaper, and actually more effective.