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

I've owned both the 38S and 380S Preamps.... I could discern very little to no difference in sound quality.  Future owners would thrilled to own either.  The are excellent controllers with wonderful transparency.  The stated improvements from fan boys and Levinson are mere marketing ploys. 
Thank you Rossman. There are a lot of 38s on the market. 380s are harder to find. If the audible difference is small between the 2 I will snag a 38s. They are going between 2 to 2500 in price. I just acquired an old 23.5 amp which still sounds good after nearly 30 yrs. A 38s should be a nice match for it and upgrade from my Parasound P7
I love my 380S.  Since these units are getting old and the case has no ventilation, you may want to consider a recap.