I’ve got the upgrade itch

I would recommend it, and the Atma-spherica, if you would like to take your system to the next level. The preamp is the heart of a system. Prima Luna is great budget stuff, but Atma-spheric is audiophile stuff. It captures levels of greater nuance… and as you couple it with better associated equipment, it will sound better and better. 
 

Save up if required. Go for the MP-3. My systems are shown under my ID.

I’m a huge fan of Atmasphere too, but you really need to commit to going fully balanced to get what their gear is capable of.  Also, I’m not sure you should limit yourself to a certain tube type.  Rather, it might be better to just identify what sound characteristics are most important to you and what improvements you’re looking for and just let that be your guide.  That said, what are you looking for?

@soix 

+1

Stretch the budget a little and spend on the preamp.  It is a crucial component.  The amp is too, of course but a bad system match where the preamp is the weak point can make the rest of your system sound , dull, bright, recessed, too forward..... whatever bad nasties you can think of can be due to the preamp.  It is as important as your speaker choice, it influences the overall character of a system 

I recently posted a similar thread and got some really good advice. You can check out my threads to see the details.