Next is what should I expect to pay. There is a set of Telefunken 12au7 on line, new for $400.
That’s high for experimenting. If you were sure that Telefunken was your favorite tube and that was a superb pair, maybe. You should be able to buy some nice tubes from a reputable dealer for $200 or less.
Here are a few good dealers. You can google them.
Andy Bouwman at Vintage Tube Services (he can be slow but many people swear by him)
Brent Jessee
Kevin Deal at Upscale Audio (some people find his personality abrasive)
Jim McShane
I hope others will chime in with those I’ve forgotten.
They will all talk to you about what tubes might work for you and they are probably all familiar with your pre. I’ve read that Brent Jessee allows returns if you don’t like the tubes. Check that with him before buying.
Take your time shopping for tubes and listen to the stock tubes for a while to get familiar with the baseline sound of your pre. Then decide what you might like to change with new tubes.
There are good sellers on ebay, but it’s not where I would start my tube journey, especially if you don’t have a tube tester.