Any of the NOS RCA valves from 50 -60s would be good. Not sure about black plates although they do get mentioned a fair bit. I just found the two RCA valves I have used in the line stage were perfect balance of tonal quality, detail and musicality.
Top man for building those stands they are superb and look very professional. I may have a go at building some myself when I retire as i am sure they would be fairly easy to build. I use some old target stands for mine which are ok and do the job but not as elegant as yours.
I have attached a few pictures of valves and my gear for you to look at.
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IMG_0024 by [url=https://www.flickr.com/photos/162281319@N03/][/IMG]
[IMG]Untitled by [url=https://www.flickr.com/photos/162281319@N03/][/IMG]
[IMG]image by [url=https://www.flickr.com/photos/162281319@N03/]J[/IMG]
[IMG]IMG_0033 by [url=https://www.flickr.com/photos/162281319@N03/]J[/IMG]
[IMG]IMG_0004 by [url=https://www.flickr.com/photos/162281319@N03/][/IMG]
[IMG]IMG_0077 by [url=https://www.flickr.com/photos/162281319@N03/][/IMG]