How to evaluate amps that are not broken in?

I'm expecting to demo Belles and JoLida amps next week at home, and both units are new. Can I assume that if I like the sound of an amp now, I will like it just as much after it's broken in? How does one account for this variable when trying to decide if a new piece of gear will be synergistic with one's system? Thanks!
they'll only improve so yes, if you like them now, you'll like them more later. my very unscientific guesstimate would be that they'll improve by about 10% - not enough to radically change your opinion of them, but possibly enough if you're on the fence.
OK, that's encouraging, Musicslug. I realize this is very much a newbie question, so I appreciate your taking the time to respond.
Break in is something that is highly debatable. Considering all of the variables that are at work EACH time you sit down and listen it is very difficult to attribute any gains directly to break in.

However, people that do believe in the theory seem to agree that only good things come the more the component is played.
I agree with musicslug that the components will only sound better over time as the components within the gear start to break in and stablize.
If you have only 2 days over the weekend I would let the gear play continously for a set period of time and then start to listen seriously;at least let the gear thermally stabilze.
Robr45, I experienced a dramatic change with my Creek 5350 se-- far beyond what I was expecting, actually. So I guess that experience puts me firmly in the "believer" camp. Rleff, I will do as you suggest. Thanks!