Agreed, a 45-70 rifle is more cumbersome than an 1892 model or Marlin in 45 Colt, and there are plenty of hot 45 colt loads that will do what you need.
Not having checked much, I'm assuming there are standards loads that should be easy to keep subsonic from a 16-18"barrel..?
This is more or less true at least what I've found making subsonic loads for a suppressed 44 mag carbine. Most weaker 44 special loads, but not all, stayed subsonic from a 16" barrel. The heavier the bullet the better, obviously.
