In the MDA series of amplifiers, you would commonly find the MDA-300 and MDA-500 come up for sale... occasionally. What you would never find or maybe once in a blue, blue moon, is a pair of MDA-600s coming up for sale (only one pair has ever appeared for sale in the used market as far as we know). Being that only two pairs were ever made - One pair for Fabio and the other pair for Dan D'Agostino himself, good luck with ever finding another pair for sale. What is even rarer than the MDA-600s is this pair of MDA-400s that Dan specially built for a long time friend of his. This could be the only pair in the World (as far as we know)! And is probably even rarer than digging up a red diamond. Only the MDA-600s and this MDA-400s has the blue power output meter on its front that are similar to the uber high end KAS amplifiers. The MDA-300 and MDA-500 do not have them.
Here's someone selling the MDA-400s
https://www.ebay.com/itm/Krell-MDA-400s-Class-A-monoblocks-1-of-a-kind-Re-capped-and-serviced-28-000...
Here's someone selling the MDA-400s
https://www.ebay.com/itm/Krell-MDA-400s-Class-A-monoblocks-1-of-a-kind-Re-capped-and-serviced-28-000...