Optimizing the Prima Luna Premium Integrated amp

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.

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.

I can't stop buying tubes even though I will certainly not live long enough to use them all. So I decided to call myself a tube "collector", this gives me license to keep a hoard of those things. I can't even imagine what I must have spent. I try to ignore that and keep telling myself I will sell them someday. But buyers can be a royal PITA some of the time. I don't tolerate the aggrivation well. So I trade some and sell others very cheaply at a loss I imagine, if I ever remember what I actually paid.
I am, currently living in exile away from most of my gear but bought a Fisher 400 receiver which is all tube. It needed a 12AX7 so did I put one of my precious 5751s in or a telefunken ? No absolutely not I bought a current production JJ 803! It is ridiculous I must admit but I like tubes so....

All 12AU7s is the manufacturers tube designation for the amp we are discussing. In that event the OP may want to try another direct sub the 5814. The 50s Hytron and later CBS Hytrons are held in high esteem but it is necesarry to them alone to get a feel for the tuybe type. It has what I call the "strong, clean signal" that I find desirable in certain situations.
The RCA 5814 versions are very good and as are many others, try em they shouldn't cost a lot. The other clean strong signal 12AU7 I have some experience with is the Brimar 13D5 which has been around for a few years, but not sure who is selling them now.
As I said earlier if you can find some good Telefunken ECC82s even used testing good you may want to try them. Lets face it most of what is called NOS is just OS and not new.
I don't have a great deal of faith in the RCA 12AU7 clear tops , from personal experience which I admit was a gift of some decent and some unusuable tubes I got in a batch. I don't know much about the others, try Brent Jessies Audiotubes web site, he has some details on the US types. I settled on 7316s a while ago as my 12AU7 of choice and have been hunting them until recently now that prices are really too high for most people. On E Betrade people list them with singles foe $100 and up to start for single tubes. I can't recommend anyone try them at that price.
Current production has gotten much better from all over. My old Pal Joe imports some of the Chinese tubes which he has great faith in. Others have admired the new Gold Lion smasll signal tubes with many other New Sensor tubes.
The 5814s when I last looked were still reasonable, I think the Brimar 13D(X) tubes have gotten a bit more expensive and Teles while Desirable are plentiful as pulls and hit and miss on price. IMHO YMMV etc. etc.

NOS power tubes are frought with difficulties. They havent been produced, in the case of EL-34s, since the late 70s at best. Finding good ones is tough but I was able to buy some Philips XF3s I believe as a quad that did look new in box. The price exceeded $100/tube so be ready for sticker shock. I am aware that some of the big tube sellers have NOS outputs but rices are commensurate with thheir rarity.

Can you sub in the New Sensor Gold Lion KT77? That would be interesting. I am kind of surprised the KT-88 wasn't good in that amp. Another "super tube to consider is the "Tung Sol" KT-120 if the trannys can handle it, ask uncle Kevy?
Finally I would make a point of trying 2 current products that have worked out well for me. The Electroharmonix 6CA7 "big bottle" and the JJ blue glass E34l (note E34l not EL34 not a typo) very underated, don't try the JJ KT-77. JJs can be bought from a webstore US based called Eurotubes.com.
I will be interested in hearing the result. I noted that no 5751 made your preamp tube list, did you try them?

Wolf thanks for the tip about a good 7025. I tried a NOS/OS 7025 many years ago. I think it was RCA, that one trial and a confirmatory message from a friend who tried another NOS/OS 7025, turned me against them.
If it's a Chinese tube it shouldn't cost too much. Your description of a clean sound is what I find in current production 12AX7s from Shuagang, very good tubes, but it is only good if it synergizes with the rest of your system.