As usaul when spending at this level I believe the differences are minor, but first the 6Ca7's were perceived to be more reliable. They are biased at twice that of the GT's. The higher bias is at the recommendation of the tube dealer/not Manley. The tube dealer states the higher bias is needed to hear the bass improvement. I agree that there is an improvement in the base region. Second, their have been posts about these amps/tubes. This last time a resistor was blown so I sent them back to Manley at their advice for a mod/repair which was covered under the warranty. (I can't say enough positive things about their tech support) To date these amps/tubes are performing flawlessly. Also I subjectively believe my soundsatge is bigger as well. I hope this is helps.