I had similar issues with the inputs on my Onix SP3 integrated. I have switched from 12AX7 to 5751 AND used attenuators.
Changing from 12AX7 to 5751 might reduce your maximum output by 30%, but it will have a much smaller difference in your normal volume range. Having said that, I love the 1950's RCA and GE 3 mica black plate 5751s in my Onix SP3.
I started with Golden Jack -10dB attenuators. Then I went to -14dBs. Now I am using EVS Ultimate Attenuators.
I still have the -14dBs with RCAs if you want to try them.
Changing from 12AX7 to 5751 might reduce your maximum output by 30%, but it will have a much smaller difference in your normal volume range. Having said that, I love the 1950's RCA and GE 3 mica black plate 5751s in my Onix SP3.
I started with Golden Jack -10dB attenuators. Then I went to -14dBs. Now I am using EVS Ultimate Attenuators.
I still have the -14dBs with RCAs if you want to try them.