So i'm understanding that what i would IT'S quite impossible..so i ask you..which headphone i have to choose to listen my favourite Music?and which desktop amp? Not only for 350euro but more too. Thank you so much for your suggestion.
This whole topic is confusing. I'll try to help a little.
You started this thread with a question about headphone amps--but also about headphones. If your 350 euros has to cover both, then my advice would be:
1 - Start by buying a new headphone that appeals to you. You mentioned an AKG 702, which may be perfect (not to my taste, but this is not about my taste). However, that AKG new may eat up a lot of your 350 euros, unless AKGs are cheaper where you live. There are many AKG models. There are also other choices that may/may not be perfect (for example, the open-back Fidelio X2's--I own & really like them. They're perfect for jazz & classical. There are also many inexpensive but good closed back headphones. There is a thread 100% dedicated to inexpensive Chinese designs that sound really good (http://www.head-fi.org/t/822184/chinese-asian-brand-info-thread-on-or-over-ear-headphones)...some of the headphones mentioned there are the Status Audio CB-1; ISK MDH9000 and 4-5 different "clones" (same design w/different brand), including LyxPro HAS-30's and Marantz MPH-2's); and various headphones by Somic.
Main point here is that you should get new headphones, which may or may not prove to be "the perfect headphone"--but you have to start someplace. Your first pair of headphones could probably be driven by the computer or cellphone while you're looking for a new or used amp. And though running the headphones w/o an amp (at least in the beginning) may not sound as good with it would with an amp, that's OK--you're starting a new learning curve here. First priority is the learn everything the new headphones can do on your music. You may eventually think this 1st pair of headphones is "just ok," or you may think they're wonderful. Either way, you'll learn about what you like & don't like in headphones.
2 - In my opinion, the amp is a different challenge, and buying an amp should be your 2nd action. Why do I say that? Because you can get a very satisfying pair of headphones for less than $100, but to get a really satisfying/strong headphone amp for under (let's say) $200 new would be a real achievement. So my recommendation would be to try to purchase a used headphone amp locally (ie, in or near your country). Likely suspects would include:
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Lake People G103 (new price = $290 USD) or G109S (new price = $525 USD). I have the very similar G109A--an extremely capable, good-sounding amp--all my headphones sound good on it
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Lake People RS 02 (new price = $649 USD) ... well reviewed
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iFi Micro iCan (new price = $349 USD). I bought and sold a used one last year: it's a very capable, flexible amp with pretty good power
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Matrix M Stage HPA-1 (only available used/a little hard to find here)...a very good, "friendly" sounding headphone amp that can also be a preamp. I just upgraded mine w/new opamp and it sounds excellent. I've owned 2 used HPA-1s...gave the 1st away as a gift.
Or you could try the
Matrix M Stage HPA-2, the update to the HPA-1. You actually buy this new on ebay for ~$190 USD. The reviews say this unit sounds is a little less warm and more accurate than the HPA-1. That makes it less interesting to me, but you may have different tastes.
I'll bet you can find at least one of the amps in this list available used (on Head-Fi or other sites) that's located somewhere in the euro zone.
http://www.ebay.com/itm/Matrix-M-stage-HPA-2-Headphone-Amplifier-AMP-black-/141685318697?hash=item20fd1a6429:g:Ft4AAMXQWzNSn~Dg
Note: I left tube amps off this list for just 1 reason: though some of them are said to be good but not very expensive (ie, LittleDot II, III, or IV; Darkvoice; etc), many users ending spending as much money on aftermarket tubes as they did on the amp (which defeats the budget!).
I really hope this helps.