Valve phono stage

Infection,  You asked Atma-sphere why "top" phono stage designers use RCA inputs.  Here's a list of reasons:
(1) It's been the industry standard for so long that nearly all tonearm cables are built for SE inputs, and "top" designers do not want to scare away any would-be customers for their megabuck products by offering XLR inputs, regardless of whether the internal circuit is balanced or not. Conversely, cable makers seem to shy away from creating true balanced phono cables with XLR termination, again because buyers are used to RCAs and because of the above, there are few phono stages that fully benefit from a balanced cable. (Although it's a good idea to use a balanced cable, even if you do terminate it with a male RCA plug, so that ground is carried on a conductor exactly like the "hot" signal conductor.)
(2) It's much more expensive to build a balanced phono circuit than it is to build an SE one, nearly twice the parts count.
(3) There is a legitimate debate as regards the advantages of a balanced phono circuit over an SE one, and some designers honestly stand by the SE type.  But anyone WILL tell you that a phono cartridge (with some oddball exceptions where the two grounds are common) offers a balanced output.  I can tell you from my years with the MP1; I NEVER have hum or noise problems with any cartridge.
(4) It's not quite true that all "top" designers used SE phono circuits.  If you want to bring solid state devices into the discussion, there are a few very expensive SS designs that are balanced; most significant are those from Nelson Pass, who I consider tops among SS designers.  We've also named some other tube phono stages that are balanced; they are not to be sneezed at either.  (I can add to the list the late Allen Wright whose all out tube phono stage, the RTP3C, is balanced input to output.)

So, for balanced tubes, we have Atma-sphere, Allnic, Aesthetix, Allen Wright, K&K, Einstein, and I am sure others.

so why do top phono stage designers use RCA inputs?

I’ll give that a shot, pending Ralph’s response.

Turntables came into being before low output moving coil cartridges became popular among audiophiles, and perhaps before any even existed. Moving magnet and other high output cartridges have much higher impedances than LOMCs, and consequently will exhibit cable sensitivity even if the interconnection is balanced. For example, as you are probably aware manufacturers of moving magnet cartridges typically provide a recommended range of load capacitance, to which the cable is usually a major contributor.

Also, when turntables came into being RCA connectors and unbalanced interconnections had become the norm on consumer electronics equipment, for a variety of reasons. So putting XLR connectors on turntables and/or phono stages or preamps would have resulted in a non-standard interface that would have ruled out the use of the RCA-to-RCA cables most people had. The additional physical space required by XLR connectors might also have been problematical in the design of some turntables and/or phono stages or preamps.

So for various reasons a tradition arose in the early days of LP playback, and traditions are often self-reinforcing.

Regards,
-- Al

Edit: This was composed before I saw Lew’s well stated response above.

What does anyone think about the Tavish Design Adagio tube phono stage?  $1790.

A few years ago  people raved about the 22 tubed NVO. Might be worth looking into.

The Tavish is awesome is as good as the LKV 2SB I compared it to,  very neutral I sent it back I wanted something more tubey if you will. I have the Rogue Ares Magnum which is very good also uses 12ax7 with 12au7 tubes excellent dynamics, warmth and sound stage