+1 for Brent Jesse, read the tone descriptions they’re spot on. Next is what tone change is best for your system. After trying about a dozen brands of NOS tubes, I’m a huge Telefunken fan. That being said the Tungsram is a less expensive way to get the Tekefunken sound, then EI would be the next closest to Telefunken. I personally prefer the higher end German tubes, followed by the NOS American tubes. I’m not a fan of Mullard driver tubes as I find they put a vail over the music. However many people absolutely love them. Also Andy at Vintage Tube Service is excellent and I’ve also purchased from the Tube Museum although they’re a little pricy but great quality. Bottom line is you have to try them to find out. I spent about $3K answering that question 5-10 years ago. Happy Hunting, Mike B.