In case anyone is interested, I've recently rolled in one of those Black Plates 1950s RCA 5R4GY Rectifier with brown base and square getters.
This rectifier tube is readily available for reasonable prices <US$40 from major online tube retailers or around US$20 if you are lucky on e-bay.
1. Inner Resolution
Compared with the Tung Sol 5Y3GT, the extreme inner resolution of the RCA 5R4GY blows the 5Y3GT away.
I can hear the faintest whispers easily in the recording studio or venue for live recordings.
For e.g., the "one, two, three" near the beginning of Brad Paisley's "This is country music" is more distinctive.
It was already present when using a 5Y3GT but I do not have to strain at all to hear the faintest whispers.
BTW, I replaced the Lorenz PCC88 input tube with an Amperex Pinched Waist 6922 for the improvement in midrange and top-end extension.
However, I lost that magical inner resolution of the Lorenz PCC88.
With the 5R4GY, I have gained back the magical extreme inner resolution of the Lorenz PCC88.
If you are the type of audiophile who wants to hear everything in a recording, e.g. the girlish giggling from a member of the audience in one of Brandi Carlile's live recordings, the 5R4GY is highly recommended.
2. Vertical Dimension to Soundstage
Another plus point is that it adds a spatial vertical dimension to recordings.
The CSP2+ already excels in left-right channel separation.
However, when using the 5Y3 family of valves which it was originally designed for, the vertical dimension is not pronounced enough for me.
I vaguely recall Skylab having mentioned this weakness of the CSP2+ when he reviewed it.
After I added the 5R4GY, when I listened to Hotel California by the Eagles, somewhere in the beginning of the track, the sonic "swish"/ "swhosh" sound effect not only appears on the left ( as when using the 5Y3) but now seems to move in a spiral (i.e. with distinctive High-low height) fashion on the left side.
The original vertical sonic dimension weakness appears to have been "fixed" through the use of the 5R4GY.