Optimizing the Prima Luna Premium Integrated amp

You might try the Mazda/Cifte 12au7's. I swapped out Mullards for the Ciftes in my tube integrated (master sound). I found the Mullards muddy in comparison.

You might want to try the Amperex Holland 12AU7s. Be careful though as they were sometimes "only" rebranded Mullards, you can tell by the etched code. My all time favorite in any position is the direct sub 7316 an Amperex product but now very expensive,
It really sounds like a German tube such as a Telefunken would do the trick. Also you can call Upscale for help since they import Prima Luna they will know what works to bring out the qualities you want.

I have swapped a number of NOS 12au7 in my Primaluna Dialogue One with great success. To my ears the Amperex Bugle Boys are the best. I have failed in my quest for a "reasonably" priced 7316 Amperex mentioned by Mechans above. I also like the RCA Clear Tops Side Getter, the Psvane Shuguang Black Treasure. Frankly, the Mullards were not as clear as the tubes above.

You should also try swapping out the 12ax7 for some Sylvania 5751 Triple Mica black plate and or/grey plate.

Another suggestion, try the NOS Russian Reflector 6n3ce output tube (6L6). This combo of tubes bring this integrated to a whole new level, much, much better.

If 5751s are suitable for this amp then you must try them in place of the 12AX7s. I am an inveterate tube roller and find switching the tube to be the fastest easiest mod you can do yourself. Most amps will tolerate changes to the smal signal tubes, if the pinout is the same.
There are a bunch of good 5751s to try but most everyone wants the "holy Grail"- the late 50s NOS triple mica black plate by Sylvania. I use them and they are truly great, but less rare 5751s work out pretty dammned well, try the RCA white print or Command series, GE are abundantand cheap. a decent place to start, try to get their 5 Star Black Plates, The rarest of the rare are Raytheon windmill getters. They are worth the trouble but very expensive and truly rare, unlike every tube you see on ebetrade. If you just want to dip a toe in the 5751 water the JAN tubes can still be had NOS but watch out as some pretend it is the late 50s early 60s tube. Check the Audio Asylum Trader section lots of reasonably priced tubes for sale, if you can't find what you want here.