It would have made for a better comparison if you had played the the same side again as there are multiple things that could change the sound of an lp from one side to the other. I've had Lp's even direct to disc that play fantastic on one side and are mispressed on the other.
Amplifier warm up
I was plalying Boz Scaggs, "Silk Degrees" LP. After side one, I ate dinner. One hour later I played side 2. The second side sounded clearer, more vibrant with better sound stage. I have a Parasound P6 preamp and Parasound Halo A21+ amp. Is it possible that an hour of warm up improved performance?