Preamp tube

Any particular tube is going to sound slightly different depending on the amp in which it’s installed. Some of the best advice I’ve been given in this hobby is to buy NOS tubes from Andy at Vintage Tube Services. Spend a little more $ now to get tubes that can possibly outlast you and perform better than any of these new production Chinese/Russian tubes.

second Andy.          tell him what you are wanting in the next tube and he will give you some suggestions..       more than likely, you wont just get one pair from him             best way to reach him is by phone

http://vintagetubeservices.com

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If you want more of that warm sound with good bass  go with Mullard 12AX7 (not cheap)
If  you want clarity and detail with good punch and bass, the Telefunken12AX7 can't be beaten, also not cheap

Another recommendation is to contact Brent Jesse (I'm only a customer), here's link to his webpage for 12ax7s , aside from his listing of what he has available he provides some views on the sound of different brands which you may find helpful regardless of who you buy from
http://www.audiotubes.com/12ax7.htm