I use 2400 for my subsonic load. I cast a Lyman 240 grain bullet, and lube with Carnuba Red. The lube you use is very important in sub sonic loads. I would not use the three powders you listed. IMO they could be case sensitive. I am going to buy a pound of Trail Boss. I have been reading about great results with Trail Boss. I would use Blue Dot as my first choice if I had to use one of the three you listed. I do shoot a reduced Red Dot load in my 45-70, but I also use Puff-Lon with it.

You are correct in leaning toward the 44 caliber for Whisper loads. Actually the 45 Colt is a very good cartridge also.I load my 44 magnum to about 950 fps. You can get away with a little more speed than that, and still stay suppressed. Tom.