The question you may ask yourself might be, what you will define as supplement/complement? Are you looking for a similar set of sonic properties of the Rad-0 in a closed form or something that is rather the opposite for an different take on overall presentation? I assume from your list of existing headphones you kinda want something with less low end focus, since you got everything greatly covered alreadyI'm thinking of getting an xc as a closed back to supplement my rad 0. I've been pondering over stellia, vc, noire, celestee etc for months but I think I prefer how planars sound. The xc also seems to be very good value price wise
I am also currently torn between the LCD-XC and Kennerton Rögnir for my next closed planar headphone trial/buy, currently leaning towards the lighter Rögnir, but seems you are already very tolerant to more 'weighty' headphones
