Headphone amplifier just as speaker amplifier takes input signal and amplify it.
Why do headphones need amp?

By loading the line-out with small resistance (headphones), the sound may change, distort or may be simply too weak for specific headphone model. Headphone amp takes signal from line-out, with infinite resistance, therefore no distortion occurs and then amplifies it.
Good amplifier should have good stereo separation, low noise and low distortion.

Valve headphone amps are different story. Used mainly by audiophiles, they're considered to be the best sounding option available. But they're priced accordingly, vintage ones are well over $1000. The vacuum tube needs to be changed oftentimes and it's also not really cheap, yet those provide truly analog sound of the 70s.
PA2V2 is the best portable amp you can find for this price

 

Have ya try o2
http://www.jdslabs.com/item.php?fetchitem=O2Full
I heard they are great. Haven't try them since I only have ath 50 . Not sure how much improvement would I get

 

... it's idea that makes music great. So, don't worry about monitors, lot of nice producers also after years of working still don't have nice/ proper monitors. ( don't go for cheaper monitors)
Buy a decent headphones ( my AKG K141 broke after a year) be very careful about them, don't listen too long, listen it more quietly ... and worry only about music, music production, sound. DO music with your mind, think about how it looks like what you listen and you will be satisfied.

Hope this will help...

 

well with your budget....

i would recommend these

Tannoy Reveal 501a
http://www.youtube.com/watch?v=MVq4GWr7l-E

when it comes to mixdown/mastering i think there are no better in the accuracy aspect+ this budget
nowadays you can shoot them with 15-30% off. (110-130$ each)
i use them as 2nd pair besides my krk rp8

 

Behringer Truth came in my mind.

 

Shame you can`t get the Prodipe Ribbon 8`s in your country cus I`ve owned a pair for 2 years and they still get a kiss goodnight EVERY night! Beautiful, sexy, French styling with a sound that makes me feel all warm inside, LOVE `EM!

Headphones suggestions, AKG K271 mk2. I also own these puppies and like my Prodipes, I LOVES `EM!

I think the main reason I LOVES my kit so much is that I learned very early on that a fool and his money are very EASILY parted. I know it`s probably gonna be difficult for you, but, you NEED to make the effort and go demo some of the kit you are interested in. I spent a good 4 weeks popping along and listening to the monitors and headphones with music I KNEW before I shelled out a single penny. Even though those pennies were burning a damn hole in my pocket and I was DESPERATE to step up my system to some proper gear, I went with my head and not my heart.

Result? You can see for yourself above......... LOVE!

 

Don't get why everybody's stuck on AKG and Sennheiser. Don't get me wrong, I do have experience w/ both (and not just), in fact, been djing w/ HD 25-1 for years. Love 'em to bits! But, there are gems around that can compete the two, and even blow 'em out of water. Just recently got myself a pair of
AIAIAI TMA-1 Studio Headphones http://www.aiaiai.dk/store/headphones/tma-1-studio-no-mic Details, clarity and generally sound out of these is astonishing. On top of that, didn't find any issues wearing them for 4-5h sessions. The HD's got permanently retired.

... And finally to answer your question. Obviously, as the others suggested, a pair of used monitors is your best option.
When you were saying it's hard to find a 2nd hand joint in India, no pun intended, I was laughing my bun off. Your country is jam-packed by producers, and thus there have to be some online, localized joints for used music gear.
http://www.sudeepaudio.com/trading.php?act=secondSale
A guy is selling KRK Systems Rokit RP5 G2 (Pair)+ Auralex Mopad Sound Isolation Pads for about 325$ - if you can stretch few bobs.
http://www.adpost.com/in/musical_instruments/897/
Here's a pair of aforementioned Samson Resolve A5s.
Have fun!

 

^^^^^ This.

I own Behringer Truths, and I've learned their sound over the years. That's what counts.

Don't mix on headphones if you can avoid it.

 

Why not? I'm curious *yes*
I'd say the opposite, mix only with headphones.

 

Nothing wrong with headphones.
Speakers do give you a more deeper stereo image.

you can hear a few imperfection that often hide on headphones.

Also listening to music for a prolonged time on headphones exhaust your ears faster. Which might cause for you to higher the volume of certain tracks because you think it's lower than reality.

On a side note. I think panning is easier to perceive using headphones over speakers.

 

I think that largely depends on the model/price/parameters. For example I haven't listened through a pair of monitors that would give me more detail and/or stereo separation than my K701. No speakers I tried came even close. *no*
Also when mixing with speakers, there's just one sweet spot, meaning you can't move. With headphones you are free to move around. *yes*

Though about the wearing fatigue, I agree. Even the softest clamp will begun to feel uncomfortable after couple of hours.

 

I have not heard the PreSonus, but that company has some of the best customer support of any company I have ever dealt with. I would suspect that they would fall into the category of "very good for the price" because that seems to be their niche. They also have a nice recipe for Louisiana style gumbo. lol A reasonable set of headphones and some decent near field monitors---- and you can still do quality work. You just need to go listen somewhere else once in a while for reference.