Mark Levinson preamp no. 38 vs 38s vs 380 vs 380s

Good day,

can someone help me one the topic. What is the difference between the 38, 38s, 380 and the 380s. I was planning on getting another preamp so that i can use balanced cable to my Krell. First i had the MF to my Krell and then my father gave me his SF Power 1. Now i have the MF hookup to my SF Power 1.

My current system
B&W Nautilus 803
Krell FPB-200c
Sonic Frontiers Power 1
Musical Fidelity A3CR preamp
Sony SCD-1

Thanks,
Todd