Valve phono stage

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

@lewm - thank you, appreciate your explanation. 

@almarg - & thanks also, informative. 

Keith told me -

 "A good fully shielded RCA to RCA interconnect from the turntable can give great sound with no hum and noise. It is the quality of the electronics makes a huge difference in the sound."

@rsf507 - funny you should mention that as a dealer offered me a used one!