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

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?

The thing that always reminds me how subjective this hobby is is the variety of choices we have. I too have owned numerous preamplifiers and heard many times more in my set-up and outside. I am currently enamoured with a Conrad Johnson PV9a that has been upgraded with cutf vcaps bypassing the huge coupling caps, and numerous other upgrades that has taken this preamplifier to another realm altogether. Total investment including purchase price, less than 1.5k including a marvelous phono stage that is just so engaging.The soundstaging, dynamic shadings and timbral accuracy is stunning and an example of what can be accomplished with a well designed vintage product and upgrading parts.

The important thing in considering a vintage unit would include purchase price, condition and cost for upgrading to determine whether or not it is worth it. The big thing to consider is that if you can pick up a good vintage product like a CJ Premier 3, ARC SP8 or a nice 6sn7 based preamp with a good power supply, much can be done to improve performance and it sure can be worth it. I know a few guys that still use Marantz 7 preamps. Good design is timeless and better parts can realize significant improvements.  

Ralph I have no doubt that having the correct value caps would be ideal however I’m not even sure that Vcap makes the cutf capacitors of the value required, 20uF X 4. In any case the cost would be prohibitive if they do which I don’t see on their web site. If there is any smearing relative to the stock polystyrene caps, it isn’t evident to me and as stated, there was significant overall improvement along with numerous other upgrades by Bill Thalmann.

Conrad Johnson has a teflon cap upgrade but if I spent what they were asking for their upgrades one might question the sanity of going that route, it sure didn't make sense to me.

Yes Ralph, the caps are at the output driving a pair of Quicksilver amps with an input impedance of 100K. I also have used with a pair of vintage MC60's and  a Berning zotl amp to equally good effect.