I was around Criteria Studios in Miami in the -80's as chief maintenance engineer. I recall that our disk mastering consoles did have variable low end 'crossovers' that could be set to combine to mono at whatever frequency you wanted. It was never used on tapes made in our studio, but outside tapes sometimes did because of poor recording/mixing engineering or phase shift from inexpensive mixers in the record process. If you didn't watch out, you'd cut a master that had low end that either couldn't be tracked by a consumer grade turntable, or wouldn't allow you to cut at decent levels or playing time. I agree, mixing/mastering without a scope around is not a good idea, and even back in the '-70's' every recording studio and mastering room I was involved with used one at virtually all times.