Textbook RT60 for a mastering control room, mostly accomplished with natural treatments, a bit of hidden in art absorbers, etc..
But first a question (s ) to the OP about your tastes, what is your resident orchestra and where are your preferred seats in the hall ? Have you heard your Spendor in a better room or system ? what does it do that yours does not ? Congrats on the Spendor BTW, most excellent.