In short: I loved the sound of my modest system, until I upgraded my amp. Now it sounds pretty horrible. It went from a warm sweet embracing easy-to-listen sound to knives and forks trying to escape from a bathtub.
So...
1. I can just unplug this new amp (used) and sell it
If you changed the amp and it is bad now, then I cannot see why people are talking about sources, DACs, TT, the room, cables, etc.
IMO… Sell that amp and move on… or back to what you had.
(The only thing you changed was the amp right?)
With respect to warm up times, I plugged in a used phono amp which traveled 1/2 way around the world. After the 90 seconds, when the LED when from yellow to green, it sounded pretty good.
I turned it off, waited a couple of minutes, and then pulled the cover off to flip switches for the cart I am running. Another 90 second of power and it was even better.
If it sounds bad at the first 2 minutes, then 2 hours or 2 days may not help it a lot.