The 5751 has lower gain than the 12AX7, hence less impact and slam.
The important thing is how Lloyd designs the amp to utilize the different tubes and their designs.
I've never heard of anyone not being thrilled by the stock tubes Lloyd provides. (In my case NOS Rogers, which are Canadian Phillips).
You will be fine with whatever tubes the good Dr. prescribes....
I switched from 12ax7 to 5751 tubes in my Bellari HA540 tube amp a while ago and have never looked back. It is an OTL design so you are really "listening to the tube" and the 5751 is definitely mellower (the sweetest midrange you ever heard with Grado RS1 or PS500). The nos 5751's are far superior - my favorites are a Mazda triple mica and RCA blackplate (expensive but worth it). My main amp now is a Woo WA7 which is a whole different beast and offers a more detailed sound, but the 5751 is an awesome tube in the right application!