What do you know and how do you like the RCA 5963 Black Plates?

I just rolled a nice set of 1957 RCA 5963 black plates with foil D getters.  I don't have many hours on them yet, but I was surprised at the sound quality and value for the price on the used market.  

I don't know much about this tube, other than it being a substitute for a 12AU7.  What do you know about them and how do you like them?

I think these tubes sound fantastic, especially for the price.  Good ol' made in the USA 50s product.  They have just a nice touch of air on the top end with great imaging and palpable soundstage.  I'm a bit surprised as all of the European made NOS tubes sort of have this "old world, old vines" kind of mysticism behind them--so much so that an ordinary old American RCA was not as valuable.  

I'm curious to see how I like these after 25 hours or so more. 
I also have a stash of these, via Brent Jesse - his description:
“This is a nice industrial type which is nearly identical to the standard 12AU7. RCA made a nice blackplate version of this tube, Sylvania has a gold pin version, and GE has it in their 5-star broadcast line. The plate voltage rating is a little lower than for a 12AU7, but for most applications, it will work fine. It has a rugged cathode and should be long lived, similar to the 5814.”

That specially-designed cathode is said to withstand long periods of time in cut-off without damaging the tube (cathode poisoning). This made them a great choice for NORAD computers, which led to a good supply of them here on the West Coast.

  A great article on using computer tubes like the 5963 for audio use:

I use them in my TADAC (Tube Audio Designs DAC) and have, like yourself, marveled at the quality and value. They are dead-silent and seem to have an unusually long lifespan. Very satisfied with the sound they produce.
Don't know about the  5963 but I had all the different brands of the massive 805 set tubes, and the RCA Blacks were the best sounding.

Cheers George
The 1950’s RCA 5963 sounds more dynamic than the standard 12AU7 Blackplate, IMO. It’s also a low-noise tube.

The RCA 5814A is also a great Blackplate. My favorite is the 5814A JAN RCA triple Mica, a military grade tube.