Different Tubes and general purpose?

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

Tube gear manufacturers tend to supply their products with decent sounding, but lower cost tubes that they can procure a large enough supply to fulfill new orders and to have backup tubes for customers wanting them to supply replacement tubes.  Availability and price are key considerations.  That is sensible because any more exotic and pricey tube MAY, but not necessarily will, improve the sound--improvement is a matter of taste and system matching.  That leaves it up to the buyer to decide whether a particular alternative is an improvement.   Ultimately, it comes down to the owner having to either purchase or borrow candidate tubes to listen to the results.

Because trial and error is really the only approach, I would recommend starting with the cheaper small tube types.  If you are new to tubes, go slow and enjoy your amp for many months before attempting to change the sound through tube swapping .  It takes time to become familiar with the sound so that any change makes an obvious difference.  Also, expect to make mistakes--where a change initially sounds good, but in time turns out to be the wrong move.

Most people who have been around tube gear a while can pretty readily hear differences between tube brands and models.  Like any form of connoisseurship, experience matters.  If you are new to wine, it is hard to distinguish, much less reasonably evaluate which is better, without experience and experimentation.

Good luck.

Buy a book dude!!

Thanks all for the practical advice.   
I’ve had my amp for 2 years now and it probably has 50-60 hours of use, so I would have thought it is been broken in.  I will call my distributor and ask what he thinks in terms of where to start on rolling any tubes.  I’m not in a rush, so I have time to play around. 

Tubes will make a big difference. All you have to do is swap in some NOS tubes from back in the 50's or 60's to more current production tubes and you will hear why these NOS tubes are superior. As one of the prominent vintage tube seller's said, "I would rather have a good amp with great tubes than a great amp with merely good tubes." 

Anyone who tells you to use fuses, powercords, or conditioners in place of changing to superior tubes is giving you misinformation. Don't get me wrong, those things can give you some improvement too, but really good tubes speak for themselves. You should give it a try. IMO.