I broke out my Lyman Cast Bullets reloading manual, and the closest I'm seeing to subsonic loads for 300 grain bullets is 29 grains of 5744 which yields a velocity of 1376 fps, about 250 fps above the supersonic threshold at sea level. You can get subsonic with a few different powders if you go to 500 or 535 grain bullets.
The Lee reloading manual has a whole section on reduced loads. I've used the formula to build low-velocity, light bullet loads for my 30-06 with great success. I've not tried it with the 45-70, though now that you've piqued my interest I might give it a try. I'm pretty sure that you can load light bullets for low velocity in your 30-30, as well.
I still have four or five pounds of Trailboss, and didn't realize that it had been discontinued. I like the powder and it's a shame I won't be able to get more.
*** Just checked the Hodgdon website, and they still list Trailboss there, but it's out of stock. A few other sites say that it will be back in production. Of course that doesn't help you now. ****