Will a phono pre-amp make a substantial difference

To assist the OP, I am a Phonostage Enthusiast as the minimum. 

Posts shared on this forum, makes it known I have carried out extensive travelling and footwork to experience a wide range of Phon's. 

SS, Valve Hybrid, Valve Input /Output, produced as Commercial Design or DIY Design /Builds are heard by myself in large numbers.

The result being I own  all types of Phon' listed. 

I can also take a TT from storage, which I occasionally do, and put little thought into the TT's basic requirements. 

When heard through a owned Phon', the TT's shortcomings are immediately recognised. 

A little effort put towards the TT's needs changes the impression immediately. 

This requires little thought towards the electrical support for the TT, to perceive betterment is created. 

The electrical support through producing End Sound clearly shows the benefits of the changes made to look after the TT's basic function. 

IMO the phonostage is very important and can make a huge difference. The Rega MM MK is a great choice and will pair well for you. I had one with MY P3 for several years until i upgraded to an all tube Zesto Andros Audio Spirit. 

You will be very happy. 

Sutherland KC Vibe and also, get a Groovetracer subplatter for your P3. One of the most significant upgrades you do.

Since you have a NAD M33 with digital inputs you might want to consider a phono preamp that converts to digital a lot sooner in the chain.   Check out the recent review for the miniDSP ADept on Audio Science Review.
Product details available here. 
 

It is interesting how AI addresses this issue of phono stages. A few weeks ago I had a post within Audiogon that was about turntables, cartridges and my current preamp, a 30 year old Aragon 24k pre with the $250 optional phono stage. I had several comments saying that I should upgrade the phono stage, but none of them seemed to have any knowledge or experience with the Aragon brand. So I asked AI what would be a great upgrade to that phono stage, and with the exception of a Schiit 9at under $500), all the recommendations were for stand alone stages in the $1,200-$2,000 price range. AI had significant regard for the built in that I have, and it's variable input adjustability. Considering the original preamp cost with phono was less than $1,500, and I've always been happy with it except for lacking balanced ins and outputs. So I'm happy that AI isn't tuned to trying to upsell anything, but seems to be sharing somewhat factual opinions. Again, all must be taken with a grain of salt and ultimately depends on what is good to your ears. Good luck to the OP.