At this time I have a tube preamp which takes 2 12ax7's in the line stage and 2 12ax7's in the built in phono stage. The 5751 is a direct replacement for the 12ax7, and the 5751 has slightly less gain. I have used 5751's in the line stage and it can be a very pleasant addition. I don't believe the 5751 is good to use in the phono stage as the extra gain of the 12ax7 is generally preferred.
In my experience there is a large difference in using 5751's versus 12ax7's and large differences within the brands of each type of tube.
Although it does not appear to have been updated lately, there is a lot of great information about the sound of different brands of mostly vintage tubes on Joe's Tube Lore https://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html
In my experience there is a large difference in using 5751's versus 12ax7's and large differences within the brands of each type of tube.
Although it does not appear to have been updated lately, there is a lot of great information about the sound of different brands of mostly vintage tubes on Joe's Tube Lore https://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html