Is getting a new pre-amp the right upgrade path?

Hi everyone - over the last few years I have built a moderate system and tweaked it along the way. I love my Magnepan MG12QRs since I have a small listening room. They are driven by a Rotel RB 991 and a Rotel RC 995 pre-amp, both with upgraded power cords (retrofit IEC connection). Sources are a heavily modified Rega RP3 with Zu Audio DL103 and Music Hall Speed Control. Phono preamp is a Bellari VP 130 and MT 502.  I am looking at the Primaluna Prologue Premium as my next possible upgrade in this chain to give it even more air. Is this the best course of action? Or should I opt for changing the speakers to Magnepan 0.7i? Or replace the turntable with a Rega Planar 6? I’d very much welcome any input .
A tube preamp might be a really nice move. I would audition it if possible- to make sure it plays nicely with the other components.

That preamp has a somewhat high output impedance and the input impedance of the Rotel amp is maybe a low average. They may or may not match well together. It depends on the preamp’s highest spec in relationship to a specific frequency. If it only gets a little higher, the match would likely be ok. If it goes up substantially, it could be a problem.

No doubt an audition should be done.

“I am looking at the Primaluna Prologue Premium as my next possible upgrade in this chain to give it even more air...”

More “air” means different thing to different people. If by more air you mean a more defined soundstage with better definition around the instruments/vocals, I think a better or different speaker, along with optimum placement, would be the way to go. In my experience with the Magnepan MMG, I could never accomplish that objective with various combinations of tube pre/SS amp, SS pre/SS amp, or SS pre/tube amp. For me, no matter what I tried, those speakers never created that sense of stage realism, even when the soundstage was wide and deep, the placement of instruments was not there. A similarly priced “box” speaker will do better in that respect. That said, the tube pre/SS amp was the best combination for juicing up the midrange and widening the stage but still with little more air. 
Now, if by air you mean more vibrancy and sparkle on the top octaves, the newer .7 will probably be a better upgrade since I’ve been told the panels tend to get dull over time and the fresh panel could bring in what’s missing.
A tube preamp combined with your Rotel amp could add a more organic presentation and a sense of realism.  
However, the Prologue Premium preamp would not be a match with your amp.
The preamp's output impedance of 2800 ohms is too high for the Rotel's impedance of 36 kOhms. The impedance specs of this preamp makes it more suited to a tube amp.

You mentioned more air. If you mean better separation between instruments and more detail, then the power source of your system should be addressed by lowering the noise floor. eg, dedicated  line of power conditioning.

Though Ralph(atmasphere) is too much of a gentleman to say it, I will- An Atma-Sphere preamp would be a great addition to your system. I would contact him to get advice on which one would be compatible with your system. Though I think one of his UV-1's might be a good match, and very affordable. There was one for sale here, but it expired. Maybe the seller still wants to sell.
If you love your current Maggie's, I would not change them.