Easy to go lower than that.
Two things first. Forget about speed of sound. The noise from the rifle is governed by how much of what kind of powder is in the cartridge. But nobody can tell with their ear, whether it went just over or just under speed of sound.
Secondly, I have loaded extremely light with a number of different calibre rifles and have never used Trail Boss, and don't intend to. There are any number of fast pistol and/or shotshell powders that are highly suitable for very light loading. I mostly use whatever I have at hand at the moment.
When our oldest son was 6 years of age, I loaded my 30-06 down to where he would rap his arm round the stock, move his eye up to the scope and safely shoot it, without enough recoil to push the rifle back.
Another time I loaded the 30-06 so low that we shot it in the basement without ear muffs. less noise than a 22 rim fire. That load, with about a 150 grain cast bullet would make clover leafs at 30 feet.
With a 270 and 100 grain jacketed bullets, I set up a large, 600 page, catalogue with a 1 5/8 inch plank behind it.
8 grains of H110 drove the bullet through both the book and the plank and it kept on going!
6 grains of H110 went through the catalogue and just made a dent in the plank.
A 3 shot group at 40 yards with the 8 grain load made a one inch group, 3 1/4" below the sighting, when set at hunting loads.
I wanted to see how low was practical with a Marlin rifle in 44 magnum.
I set up a large, round block of wood, cut from a dense, heavy Douglas Fir log, as a bullet stop.
Using 240 grain cast bullets, 4 grains of Unique buried the bullet in the hard wood, with the base of the bullet 3/4" in.
3 1/2 grains of Unique buried the bullet, with the base about 3/8" in from the surface.
3 grains of Unique buried the bullet in the wood, but the base of the bullet was about flush with the surface.
With your 30-06 just try a few grains of whatever fast powder you have and tell us how you made out.
CAUTION--It is obvious that a bullet may become stuck in the barrel when using such light loads.
Thus, it is imperative that the barrel is checked for obstruction before firing another shot.
I have stuck almost none and when they did stick there was no sound! Lead bullets are very easy to push back out, but jacketed may require standing the rifle muzzle down and pouring something like kerosene or Hoppe's #9 down the barrel and tapping it back next morning.