I have rolled small signal tubes quite a bit, especially 6DJ8-E88CC-6922-E188CC-7308 types, and found that a tube's "flavor" shows more or less immediately, in minutes, but the flavor usually develops, so to speak, over the next 5-6 hours; sometimes more, but I can't remember a small signal tube that's changed much after 12 hours.
I wonder if the process with tubes is really a "break in" or a thorough "warm up." Tubes heat to operating specs quickly when they are used in a component, but NOS tubes have been on the shelf for quite a while, so I wonder if they're really just waking up, as it were. The time may vary with whether the piece is designed to be easy on tubes or not. 'Not sure where your ARC piece falls in that continuum. The difference between break-in and warm up may be semantic, but maybe not. With tubes, I've never heard the kind of fundamental change that I've heard with other components. When I first played my Virgo 3s, for example, I thought I had made a very expensive mistake, but one morning I woke up and they sounded great. Similarly, anything I've owned with Blackgates hasn't shown is most developed mellifluousness until it hits the 200 hour mark -- and then some. For me, at least, I've never heard that kind of changes in tubes. Again, ymmv.
But Majicjazz may have hit the nail on the head. Tubes branded Mullard or Amperex may actually have been made at the same factory. The way to tell is by looking at the second line of the etched codes on the side of the tube, near the bottom. If the second line begins with an asterisk (*), then it was made at the Amperex factory in Hicksville, NY. If it starts with a sideways triangle (almost like a delta symbol), then it was made at the Philips factory in Heerlen, Holland. If it starts with a B, then it's from the Mullard factory in Blackburn, UK. It's very common for tubes branded Mullard or Amperex to have been made at the Heerlen factory. If your tubes sound the same, that may be the reason. My .02, as they say. Hope this helps.