Should I use an external Phono Preamp

@mooks005 The ST-70 is an excellent SUT and will serve you well no matter what you do in the future.  I recommend that the extra phono cable you use with it be a match for the one you already are using and that you keep it as short as possible.  Also, in spite of lewm's assessment that my advice is "poppycock", he is entitled to his opinion after all, I stand by my advice that once you settle in with your new SUT, you go ahead and try that E-07, which I believe is well worth your consideration.  In case it is not obvious, the E-07 would also work just fine with with a transformer.  I have two external phono stages and use them both with and without transformers.  It all gets down to taste and what works best with cartridge selection.  Keep in mind too, that you and I share essentially very similar turntables, mine is the GAE version.  I also have other turntables.  Finally, like you I have an excellent built in phono preamp, mine in a Marantz, your Mac is excellent no doubt, but only by comparing them side by side can you determine if the E-07 is better, and if so enough better to be worth the cost to you.  There has been a lot of conflicting opinion expressed in this discussion.  Do not let that distract you.  

I also ordered two Ortofon 6NX-TSV-1010R cables in 1 and 1.2 meter lengths. This will allow me to compare vs the Valhalla 2

I know the Ortofon ST 90 and would own one today if discovered before the now long term owned Hashimoto SUT, that was commission built for me.

A recollection of a later comparison, I recall a close performance between the two SUTs, but the price difference between the 90 and the Hashimoto was a few hundred £'s extra for the 90.

Back then I vaguely recall, I felt the ST 90 was an upgrade, but probably influenced by my usage of Ortofon MC's.

Prior to and following the use of the H'moto in the home system, I was receiving demos and being loaned a few SUT's to home trial. I was quite content when discovering the H'moto and this remained in place for many years until I superseded it.

When trying out the options on the new SS MC/MM Input Phon', the H'moto has taken top spot to be used with it. Never say never. 

I suspect the Ortofon sound to remain with the ST 70, and do not feel an overly rich in tone sonic trait will be perceived in the end sound. 

Looking forward to your sharing your thoughts on this new encounter.  

I find it interesting that even higher end/higher priced phono stages benefit from a SUT. I’ve also noticed, and this may be a coincidence or it is more users of tube phono stages, that post that they achieved a good improvement with a SUT. 
I recently tried a tubed Lab12 Melto2 phono stage and overall I liked the sound but I couldn’t fully appreciate it due to surface noise that was considerably higher with the tubed phono than I was used to with my solid state unit. I sold the Melto 2 and the gentleman who bought it is using it with a SUT. He loves it. 
I went on to upgrade my solid state phono to a model few steps up the line.
There is even less surface noise now and overall it sounds excellent with my Hana Umami Blue cartridge. 
 

I am not against tubes, and I haven’t tried everything under the sun, but it seems to me that tube phono stages probably benefit from the SUT the most.

I would not consider adding a SUT to my solid state setup - I’m just afraid to compromise the performance. 

I use two SUT's selected after hearing many SUT's both before and after my selections are made. Also I have in the past loaned a few versions of a Head Amp design.

All are used with Phonostages in use prior to the most recently added custom design SS Phon'.

The new SS Phon has been extremely impressive through the MC Input, but curiosity led me to use SUTs on the MM Input. The outcome being the SUT, not expected to be the better of the two, proved itself with no doubt to be the better interface with the SS Phon's MM Input. The SS Phon's MC Input is still the superior end sound, at this stage of early usage.

Time to call out for the loan of the Head Amp designs in their entirety, to satisfy the curiosity once more.   

Maybe a reverse riaa will be used to put a good injection of usage into both of the SS Phon's inputs before any further comparisons. 

As stated on many occasions on the Gon:

Comparing is the Thief of  enjoyment   

The sooner the itch is scratched, the better.