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

There are many new and used preamps in $2000~3000 range.
First, some questions; what amp are using, do you want balanced or single-ended (RCA) input/output? Do you seek a warm, "tubey" sound or more neutral and transparent?

You may not need to go back 10-15 years for used. It depends on the quality of your components you are adding to. For example, Your budget could get you a nearly new Conrad Johnson...
https://www.audiogon.com/listings/tube-conrad-johnson-et3-se-line-stage-preamplifier-2017-01-23-preamplifiers-12601-south-road-ny

or a new Atma-Sphere pre with phono stage...
http://www.atma-sphere.com/Products/#UV-1

or an amazing deal on an ARC...
https://www.audiogon.com/listings/tube-audio-research-ls-17se-immaculate-7mos-old-2017-02-12-preampl...

IMO, a 10+ year old preamp is a good value, but realize that it will need to have some internal parts replaced due to age. So add about $300 (plus tubes) to the price of an older component.

I think it certainly can be a good decision if you purchase for the right price taking into account tube replacement and replacing/upgrading parts as noted by lowrider. BTW you mention the SP15, do you require a phono stage?

For $2-3K you can get a really good used tube pre, especially if you don’t need a phono stage. But even if you do. Not to disagree w my esteemed colleagues, but most tube pre-amps (with some notable exceptions) are very easy on tubes, so I would not necessarily believe that you would have to budget for replacements. Even if you so, there are many good new stock small signal tubes available w/o breaking the bank. There are lots of good ones for sale now; in addition to the CJs and ARCs, there are models from Audio Note, BAT, and Modwright available well within your budget.

BTW, it's my opinion that a good pre is the heart of a good system, you can't go wrong here with getting the best you can afford. 

I like my ARC LS-25 Mk II, which ocst $2500 with new tubes used from a dealer.

You can get different sound than ARC, but it is not clear to me that you can get better sound.

Other options include CJ (phatter sound)

or maybe Primaluna tho cost may not fit your requirements.

I like to stick to companies with established track records, good support, and a history of long development towards some euphonic goal.

I've owned a melos 222 preamp for about 20 years. I had it looked at last year and upgraded some switches, the volume pot, umbilical cord, added an IEC, etc., to great result. Although it's not the level preamp you're looking for, my point is that it's worth upgrading old quality equipment, if you like it.