What should I hear as my amp breaks in?

I find most good equipment seems to get a bit more open and relaxed after a weeks play or so, but the amps essential qualities are apparent immediately out of box

Excellent thoughts. I have the older preamp and could put it back in. You are right. I'm not an audio reviewer with some equipment to analyze. I just need to close my eyes and get lost in the music. We all here have a thing for music and probably get more of a dopamine buzz from our music than the rest of the poplus. I think we have a gift that others don't understand
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@blueranger 

I'm still hoping for that little extra

It sounds as if the new amp already surpasses what you heard from your former amp so what further improvements are you hoping to hear?   I am asking because I have owned amps that "checked all the boxes" but simply didn't give me what I was hoping for.  So, I believe each listener has intangible preferences that push their buttons when they are right.  I have had gear that gets me most of the way there but not quite.

Since you lived with (and presumably enjoyed) your previous incarnation of the same amp for so long, and since you have indicated the new amp has brought further improvements over the former (already well-liked amp) then I suggest you quit listening so hard and simply enjoy what you are hearing for a while before being critical about the parts.  The dedicated line and improved power cord should have moved you in a better direction.  If you want to try something, put your former preamp in the system and listen (assuming you still own it).  I have found that preamps can have a significant effect on how a system sounds so maybe running the new amp with the former preamp will provide better context on the sound of the new amp. 

+1 @mitch2 
Unlike others here I've only owned a few amps over 40+ years in this hobby (Pioneer, Denon, PS Audio, NAD, Krell and currently Vincent). I agree w mitch2 that I've also only heard small improvements in SQ as the hours add up. IME mine all sounded a little rough for the first 15-20 mins (about 3-4 songs) and then they smooth out. Honestly, I can tell a lot more differences in SQ between recordings than I ever could between low-hours and high-hours play time. Just my 2c, YMMV.
Enjoy your new system! I bet it sounds fabulous!!!

Tom

Thanks. Bryston does some kind of electrical load on their amps for so many hours for quality. I don't remember my otber Bryston changing much but that was 15 years ago. But I'm still hoping for that little extra. I have 6 new things in my system in tbe last 10 days. This amp, 17b3 preamp, SA-10 SACD player, power cord and a 10 ga dedicated line. Plus maple isolation. I'm out recovering from surgery so I'm intensely listening. I also wonder if I might miss small and gradual changes like someone leaving tbeir system on and only checking on it every couple of days. Thanks for sharing your experiences.

@blueranger

Immediately I noticed the music came out of a darker background and the music was sweeter and vocals clearer.

What you are hearing is good.  You should indeed expect to hear immediate improvements with your new amp, particularly since the new 14b3 has received excellent reviews. 

If the amp doesn't sound good right out of the box, IMO no amount of "burn-in" is going to change that.  I have owned a bunch of amps and have never heard the dramatic swings in performance that some here talk about. Once the capacitors are fully charged you should be well over 90 percent of the way to the final voicing - IMO.  I wouldn't argue with folks who say there are minor improvements or that a touch of initial edginess may disappear after being powered up and playing music for a time (like a week or less) but with the 10 or more $5K - $20K amps I have owned (a couple of crappy ones but most very good), I have never once heard anything remotely approaching dramatic changes after hundreds of hours.  Enjoy your new amps.