Will a phono pre-amp make a substantial difference

I would definitely go for the Fono MM MK5 and I especially like your choice of the 2MR Bronze with your P3 which in it’s original iteration has been around for a very long time and is highly regarded, also with the fine line stylus will track and render groove information better with lower noise than many cartridges in it’s price range. You certainly have been very thoughtful about your source and amplification choices and getting the most bang for your buck. Way to go!

Take a look at the Rega Couple 3 interconnect for each of your connections to your amp. They are made with Klotz cable and Neutrik connectors and are very well assembled and will be good enough to use in any future upgrade to your system no matter what level you decide to take it to. I believe that’s what’s on your P3. Food for thought.

It is also worth looking into how the TT is supported?

How effective is the present mounting able to Isolate the TT from being effected by local ambient energies being transferred within the listening environment.

The stylus creates a mechanical energy that is converted to being and electrical based signal to be amplified / gain added.

The Phonostage is extremely important, firstly for the basic reason it is the first device to add gain to the conversion to the electrical signal.

A correctly thought out mounting methodology for the TT, will substantially assist with the Sent Signal being extracted info from the LP only. Hence, the gain added by the Phonostage is to a signal sent not containing energies / minimised unwanted energies that create an adulterated signal.

A Phonostage is a crucial device in developing the Sources sent signal to become an End Sound. The Phon' has a much much improved influence when enabled to process what is a minimal adulterated to pure sent signal.

I stand strong in the notion the expensive to purchase Phon' receiving a poor produced sent signal, is a less attractive End Sound, than a cheaper Phon' receiving a Signal that is close to being unadulterated. 

I am not familiar with the Rega Phono Preamp--although I have their Brio integrated amplifier, which is excellent--but I know that upgrading from the bottom Schiit phono preamp to a Lounge Audio Gold Phono preamp made a huge sound quality improvement when playing records back on my system, which consists of a THorens TD 126 MKII, Audio Technical 540ML cartridge (MM), the Loung Audio phono preamp, Rogue Audio RP-7 preamp, Odyssey mono bloc amplifiers, and Fyne F-702 speaker and two Rythmik 12Se Subs.  Some of my late 1970s and 1980, and 1990s audiophile LPs make it sound as if the performers are right in my room.  So, a quality phono stage can make a huge difference in sound quality.

Having a phono stage with on the fly adjustable gain is a huge plus. And one with a separate power supply is even better. 

pondac +1

By far the most significant upgrade I made to my vinyl source was moving my Linn LP-12 to a separate wall shelf.  Rega makes a wall shelf dedicated to their turntables. Take the hint 😆