I am sure Martin said that his suggestions on the arrangements always hit a brick wall with the band. He was however a master on capturing the best performances from each member and of course also getting a crystal clear and dynamic sound
It would be interesting to find this interview.
However I think that Birch had much more say than Kevin will ever have. The reason is simple, he had a legendary status already when was hired by an aspiring band. I think he worked exclusively for Maiden too, that speaks a ton.
Kevin on the other hand, had made recently a huge album (I believe
the only breakthrough album in his career) with then teenagers Silverchair -who oddly didn't hire him back for their second album- but he was not even remotely the legend Birch was.
My impression is that Steve wanted a good engineer but not someone that will change him, the way Bob Rock did to Metallica. And he didn't want someone to particularly say no to him.
Yeah I can imagine the Unforgiven solo situation between Bob Rock and Hammet/Urlich in Maiden.
Birch ranting something to Murray about his solo, Dave smiling from ear to ear and thinking what the fuck is this guy on, while McBrain interrupts with oi, if you think you can do better take the fucking guitar then you wanker
All kidding aside, I was driving a 300-ish km today and naturally I was listening Senjutsu. But this time after each Senjutsu song I was listening a "similar" Black Album one. (i.e. Darkest Hour -Nothing Else Matters, Wherever I May Roam -Stratego etc)
After a few comparisons, I realized a simple truth that escaped me so far:
You cannot "produce" a band who doesn't want to be "produced".
You can insert other verbs to the above such as change, manage, guide, etc. It's the same principal of openness and utter trust towards your producer, mentor, manager, guide, you name it.