It is a good idea to purchase a vintage tube pre-amp.

Since the new one are pretty expensive and what I can afford is an old one with budget of US$2000~3000. Since I don't have much knowlege on electronics, but I have authentic listening capabilities. Is it good idea to buy an old gear of tube pre amp. around 10 or 15 years such as Audio Research SP-15 or something similar (the newer one within my budget is better of course).  I would prefer a stable, long time use gear without much trouble.
No need to buy a vintage tube preamp when Don Sach's is offering a SOTA tube preamp, based on the venerable 6SN7 octal tubes, for around $2K, plus/minus.  I got one of his prototypes and have had him incorporate his upgrades to his current offering, and it is a stunningly fine preamp.  Google Don Sachs and look at this line stage. It is by far the best preamp I have ever heard in my system by far and I have had more vintage tube preamps that I can recall.  I reckon I could not improve on its performance for under ~$10K.  Good luck.  

I have some vintage  mac's   ss and a Fisher 500c  tubealong with some Dynaco st70's and wm5's they all sound great but these old tubes are nice.
For 1500 to 2000 you can get a nice Fisher that's bee completely restored.
Mine is hooked to some vintage altec's and sound  they great.
I know that's not  slick audiophile gear but it will put a giant smile on your face.
I have some audiophile friends I recently met, we've been  getting together aND listening to one guys high end gear.
It's  very nice , very crisp all the right things.
After listening to some Pink Floyd on his I came home to the 500 and heard what PF is supposet to sound like.
His just seems to have no bottom end, I'm  no bass guy but it just did not sound right on his.
I loved my old MFA Magus. Not in good repair at the moment but when it was good it was very good. I'm looking to sell it if someone wants a worthy project. Or I'll get it up to specs and then sell it. I had issues with my power amp so switched to an integrated to simplify my electronic life. 
I previiously had an Audio Research LS26 which sounded superb.  Plus it is both single ended and ballanced.  Should be able to get a good used one for around $3000.  Can not do better for the money.
I have a Zesto Leto just like the one listed here on Audiogon. You can run it balanced or single ended. It's not the euphonic tube pre of vintage make but the company makes first rate electronics. The output impedance stays low so a long run to your amp is not a problem. It's on here for $3500 but you may be able to bargain. I have high efficiency speakers and you can turn it all the way up, dead quiet.
I paid a lot more for mine and would buy it again.