Newb Tube question

I have a ProJect Tube Box DS2 which came with JJ ECC 803 tubes. Sounds quite decent with stock tubes. Its construction is different from your Tube Box S2 - the tubes are sunk inside, and on order to replace them with NOS tubes, I would have to open the top cover, remove the stock tubes together with heat dampers glued to them and install vintage tubes. I could not cope with this simple task as I could find a suitable tool.
Re long vs short plates: It is rumored that tubes with long plates (such as Mullard tubes) have higher value and sound better, but I would say it depends on the application. 

Not long ago I bought a quad each of Upscale Audio’s Platinum Grade for the following Russian re-issue tubes: Mullard, Tung-Sol gold pins, Gold Lion. For use in my VAC Renaissance phono stage. All of them sound really good! But yes I slightly prefer the long plates (Mullard, Gold Lion) to short (Tung-Sol). The Gold Lions are particularly nice. The Mullards are great for systems that need help taming an aggressive treble response.

I also tested these in my MaxPreamp2 tester, and yes both long plate types show notably higher transconductance than the short plate Tung-Sol (+10% - 15%). The Tung-Sols weren’t weak, though - in fact they’re still a good deal stronger than most of the vintage/NOS 12ax7 in my large collection.

That said, even though they measure a lot lower in transconductance, my favorite vintage 12ax7- the 1960’s Mazda chrome plates - still whip all 3 of the the new tubes in detail. I’m currently using a mix of Mazdas and the tall plate Russians, since too much Mazdas causes too much top-end energy.

+1 Tomcy6.....great answer!

I moved from NAD PP2 to the ProJect Tube Box DS.  I would imagine the sound is similar to your model.  That move was dramatic in my system.  I have Audio Research amps and preamp so later upgraded to the PH5 which was another wonderful leap in sound.  It seems we are similar that we simply love what the tubes offer...in general.  I have had minimal experience with tube rolling and have not been able to distinguish or see any appreciable differences.  You have options for a minimal investment so I would encourage you to look at a different mfg tube.  You get into the NOS realm and you can easily spend more on a pair of tubes than you did on the preamp.  I feel it would be a better investment to upgrade the component to whatever brand preamp speaks to you.  Shop the used market and you can always resell if you don't like it.

Having recently acquired a Pro Ject Tube Box S2 phono stage,

I'm liking what I hear

and am wanting to learn more about tubes.

No no no, it's a bottomless well, stick with whatever you already like, sleep like a newborn baby.