So, i have a 96 era SM-400s with the ORIGINAL GT-12AX7. I decided that mebbe i'll take my tech's advice and swap tubes. afterall, it's only been 12 years. I walked out of the music store with a GT-12AX7-R for 21 bucks. I A/b ed them and i will post a review after further testing, but after a 20 min trial, these are my impressions. 1. warmer mids. I wouldn't go as far as to say it sounds any bigger or anything... i think it's slightly (perception) louder because of it... we will see how i feel tomorrow. 2. much clearer. I have a whole new appreciation for harmonics... even my girlfriend could hear a marked diffrence. 3. increased top end definition... similar to above except for notes stay... sassy now on the top end... previously they tended to get a little sterile. 4. I fell that the tube is slightly quicker to compress than the old tube... I hate to say it but i do like it better that way. all in all, i'm gonna leave it in for the time being. i like the character. it has more voice than some other preamp tubes, so it may not be your thing.