I read another advisement that 12AX7s sound the same- not. I have 40 years experience in the field of home entertainment high-end audio with modifying amplifiers for best sound in home entertainment amplifiers. This is my hobby. As a tech, I can repair anything of audio application plus receivers with regard to vacuum tubes.

There are so many brands available that a small book could be written of the 12AX7 family of tubes. The circuit itself that uses a 12AX7 and the next tube downstream greatly influences the sonics. In other words a certain 12AX7 may perform better in one application than another for the absolute best tube. On the other hand a great tube rarely sounds just average in some circuits. Further, at times one manufactures tube compliments the following amplifier tube in an amplifier or preamp. A perfect example was an early Amperex 12AX7 used as a phase splitter/amplifier to PP EL84s seem to fix the somewhat higher level of distortion that caused some displeasure in my highly modified Kenwood tube amplifier. (I never did care for the Kenwood & sold it). Anyways, unique situations makes it difficult to recommend just one 12AX7 tube with regard to best sonics for all applications. Plus, each person has certain sonic needs that can be unique to that individual.

Three tubes quite worthy are 1950s to 1970s Brimar CV4004 (12AX7), Mullard 12AX7 tall (long) plate and Sovtek 12AX7LPS for a special reason.

The Brimar 'box plate' are the most accurate and sonically neutral tube with open & airy sonics. The tube is simply very low distortion and my favorite 12AX7. Explaining a 'box' plate is one side is crimped together creating a fin and the other side is a square box shape.

The Mullard 12AX7 tall plate is the most musical 12AX7 I ever heard. I believe the tube was manufactured from about 1950 to perhaps as late as 1960-61. The Mullard long plate offers a crystal clear and airy sonic presentation with some midrange bloom. The sound is simply charming to my ears like a sexy lady is to ones eyes. The Mullard 12AX7 short plate is a different tube, so be aware.

The new manufacture Sovtek 12AX7LPS is likely the best new manufacture tube with regard to price/performance available for home entertainment amplifiers. The 12AX7LPS does not sonically compare to the Brimar or Mullard above, but is unbeatable for the price. The tube has a special 'twisted' filament design for low noise operation and can replace 7025 tubes that were designed for low filament noise operation with regard to AC filament noise (hum). As one person stated, 'an honest tube'.

One issue with the vintage Brimar and Mullard long plate is the high cost. The reason the tubes are so expensive is the performance is simply excellent and demand is higher than supply. It is more than just 'name chasing' desires regarding the Mullard & Brimar. As a reference, I have tested about thirty to perhaps forty various 12AX7s in many audio circuits over a period of 40 years. 

Keep in mind, my experience with guitar amplifier applications is limited. I like low distortion, neutral & accurate playback audio. I also like distortion in guitar amplifiers. Simply, I want to hear the music in my listrening room that accurately represents the original performance.