Hi Ron
The Hivac CV491 KB/HR ( in gold lettering) in black plate are special sounding tubes: They are warm, yet sounding clear in a sweet kinda way, abit like the black plate, square getter brimar tubes of the same vintage. They sound less "bloated" than some mullards in certain systems. If you like the sound of the Hivacs, there is no replacement, there's a uniqueness to tits sound profile,
The Longplate, Sq getter Mullards circa around1959 are nice, its got all the signature warmth of the mullards, but without the bloatedness of some later productions.
The Black plate versions are even better, but they are "unobtanium", so don't bother!
Hope this helps