Should I use an external Phono Preamp

I have also looked on Amazon, where I discovered that in the UK, a range of MC Step Up Transformers are offered, which will all be returnable and fully refunded.

Similar should be found on the US site, with the same refund-on-return sales service. 

So, we have some knowledgeable guys testifying that the Mac owned by the OP is a very fine piece of gear and would not be easily bettered except by spending a great deal of money, and yet others suggest that a SUT, just any SUT, might be an upgrade to the high gain circuit of the Mac.  Then we have another respected contributor saying that no full function preamplifier can compete with a separate phono stage when it comes to vinyl reproduction. ("Poppycock" is what I say to that.) But what is a poor OP to do with such confusing advice?

@mooks005

Here’s your chance!

https://www.musicdirect.com/phono-preamps/luxman-e-07-mm-mc-phono-preamplifier-open-box/

I used to own a Luxman EQ-500 and replaced it with a Moonriver 505 phono and couldn't be happier! The MR is superior in every way. I had outfitted the Luxman with Amperex NOS tubes as well. Haven't heard the E-07, so I have no comparisons.

The OP is expressing their desire to have a trial period of dabbling with an alternate end sound able to be produced with their system.

The OP has already intimated they have undergone a period of Tube Rolling and exchanges seemingly have occurred, as the OP stated

"I have swapped out tubes. I am using Mullard NOS tubes in the phono pre-amp stage and Telefunkin and Siemens in the line stage. I am not complaining about the sound at all. Just always looking for improvement. I may try a new tonearm cable first and then test a SUT ". 

The only way to discover improvement is to 'hear an improvement manifest' in the end sound. Not the math, circuit or topology, which designs how electricity will be transferred, can be the sole cause of the route to improvement. As none of those elements in audio is sound being produced. Audio equipment selections are almost always made on how Electrical Energy, when converted to Kinetic Energy, generates emotion within a listener. Multiple individuals exposed to the same Kinetic Energy are not unanimous in their attraction to the sound being created.

Standalone Phonostages as a Phon' Rolling exercise is as prohibitive due to potential costs as is Tonearm Rolling, Turntable Rolling, Pre or power-amp rolling, Speaker rolling.

Suggestions being made to the OP to produce a different sound to be assessed where the costs are much more friendly to an individual is not bad advice, especially when the OP is keen to encounter new end sound produced within their listening space.

I certainly am not the one to suggest the OP lays down their desire and puts up with what is. No, no, that would not be my being 'True to Myself'.

I hope the OP discovers what they are hoping to achieve. I also hope this same desire remains and their curiosity about a Tonearm is also investigated and bears new fruit to be picked as well.

The story sounds like a fledgling journey of discoveries to be made.

Hopefully the OP is met with results similar to those I found, where all that was required from me to make these discoveries that ultimately enforced a rethink and subsequent equipment change was a willingness to express curiosity. 

Pindac, So I guess the way to go is to riddle the poor guy with bits of conflicting advice, leaving him at least as confused as he was when he started. In one aspect, I do agree with you; each of us has to figure out for himself or herself what is the way to audio satisfaction, and that can only come about by personal trial and error or trial and success.