Some interesting insights here that bring up thoughts for me about tube conversations.
As Joman points to, the rectifier does its sound thing based on how it effects the following tubes. Taken further, he also points to how the UFO2 and UFO25 will reveal tubes differently, which to me points to how everything before and after the rectifier, power, power supply design, connectors, cables, source, room, other signal path parts and tubes, etc....all effect the rectifier's effect. This variability alone could make a given really good rectifier great here, and not as great there.
No less influential on the sound of a rectifier, it takes a fairly worked out system/room to reveal tube qualities clearly and with relative accuracy, presenting a vaster labyrinth of variables in how "good" or "bad" a given tube can appear in different settings. Every single thing that makes up a system room, and how those parts, and components are tuned to balance, cumulatively effect what tube sounds better or worse in a given system/room.
Making it more complex, as Joman points to talking about different vintages and makes of GZ34s....tube types are not enough for serious tuning, but they can be a good pointer to certain characteristics. This is clearly not just rectifiers, but all tube type's have sonic characteristics that vary due to design, construction, parts, vintage, companies, and specific factories.
That a rectifier can create a pretty profound change in a relatively revealing system/room remains true...but which is "best" or "worst" among good tubes will vary depending on how it fits into individual systems. Joman's experience with the National PCC88/7DJ8 going from not so good, to good with a rectifier change illustrates this. I have had similar experiences many times as my amps have refined, lately, about all decent tubes sounding great in the right company, but like Joman, still, I have my favorite combinations.
I think this points to part of how certain tubes rise to higher consensus, like Mullard GZ34s, especially late 50s metal bases. They are costly because they fit the systems and tastes of enough people that are willing to pay that much for a rectifier...Similar is Orange Crush's Brimar 5R4GY, another popular tube for that type.
Yet we all find our preferred choices in tubes based on our system state of development and specific balancing needs in the moment of trying new tubes. And the particular progression that brought us to trying that particular tube, and how it works, is variable system to system...again, a given tube having characteristic qualities, but variability depending on setting.
As an example, I have
yet to keep in the few late 50s/early sixties GZ34s I have (one Mullard with 5 notch plates and double O getters, one Holland Phillips, smooth plates, double D getters). I really like the tubes, dynamic, revealing, extended, great solid bass, pleasantly warm, textured, smooth...rich and complete sounding tubes....And they do sound pretty different, but both have Phillips-made GZ34 characteristics that are identifiable. Here, for all their beautiful qualities, without tuning carefully to them, they shift my balance a bit too much into slight heaviness, where GZ32 types, Type 80, 5Z4s (especially specific ones), Sophia 274Bs, and others fit more easily. This does not make the latter tubes better, just a better fit for me. Likewise, I have
so far found early 60s Amperex 7308s compelling, but finally they have a bit too much "of a sound" in my setup....Very articulate and spacious in warm and musical ways, with great bass, really nice textures and space, mostly excellent balance....but.... I think it is the smooth top that goes too far into roll off for me, reduction in very fine detail making the midrange and top sound incomplete to me. I am pretty sure that more fine detail information would make me love the tube, but it isn't there. Yet, like good Euro GZ34s, the American Amperex is a very well liked tube by those who's systems and tastes they fit well in.
We could write this sort of thing off a lot to tastes, and there are no doubt cases where this is true. But I would suggest that many of us, given a chance to hear, say five really seriously refined Decware based system/rooms, we may well like/agree for the most part with these systems, though they would have developed differently to "get there." Even when we did not like some parts that make another system based upon how those components or parts fit in our own system development, we may well like how someone else made them work in their efforts toward beauty and balance.
Leading to my request. In the end we will likely choose different progressions of parts to make a system whole, each decision that sticks creating the foundation for how we choose to tune the system further. So at a given stage of development, one might use a tube to tune for a touch more warmth to reach balance, while another might be tuning for more transparency and neutrality to reach an ultimately similar balance. Opinions about these two tubes being "better" or "worse" in a given system, where their different emphasis could work very well for different balancing needs, could easily lead to conflicting information for others about how "good" the tubes are.
But, if the systems are revealing, describing the characteristics of a tube sound, and direct sonic comparisons, can be pretty telling for other's analysis in their pursuit of improving their own system. Regardless of system/room configuration, describing characteristics that cause us to prefer one tube over another can tell a story that can be more easily applied across quality systems requiring varying treatment. It can give a point of reference. Whereas, opinions without descriptors and comparators, can help at times, but may not be all that useful toward cost efficient ways to enhance a complex system/room with different focus or needs.
Even then, as all of us who have explored tubes have found...among really good tubes, what is great there, is not necessarily great here. But with relatively complete information about the sound of a tube compared to others, hopefully we can make better approximations. Tubes are so fun!