Just a suggestion: NOS Mullards are typically the best sounding (according to most opinions) and longest lasting rectifiers, extant. Especially; those from the Blackburn plant. ie: http://tctubes.com/Mullard-EZ90-6X4-tube-1.aspx and:
https://www.tubemonger.com/Mullard_CV493_6X4_MINT_NOS_Blackburn_Gt_Britain_p/1212.htm Don't know if there's much diff in sound quality/presentation, between the D and square getters, but given the low prices; I'd try both..
