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

Don Sachs upgraded my HK C-I and it was splendid, but his own preamp, mentioned above, is far better.  I have had it for a couple of years and with his latest improvements, it is peerless.  There is magic in 6NS7 tubes!!
Mark

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.  

@tubegroover , just FWIW, bypassing an existing coupling cap is often a bad idea. I don't doubt that you enjoy how the unit sounds now, but I strongly suspect that if you had cutf V-Caps of the correct value as used in the stock cj that it would sound even better. It is often the case that bypassing coupling caps can result in smearing (especially at higher frequencies); for this reason it usually seems best to give the coupling caps your best shot with a single part.

The problem is how to keep good maintenance of the vintage gears.  Some parts are hard to find, some changes might change your sound reproductions. Also, judging on it's rare quantity, it is hard to find the right, experienced, dedicated engineer to fix it.  

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.