Electronic crossover and later, x-rare, addition to the ESOTEC line, like the MC-1000 or BTL-1...
The Ad-6 was not built by Marantz in Japan but at first by Gekko Laboratory then by Exceltronics in Japan ; there was an Ad-5, too, before the 6, therefore sans Esotec tag but made by Chubuec...
Sony was ruling the market since 1978 with the TA-D88B and ESPRIT TA-D900 later on, with Accuphase and the Pioneer D-23 playing on the remote side (except in Japan) and whatever market space was left.
So the Ad-6, introduced as late as it was, couldn't sell much. In fact it didn't sell at all. Bar a block-diagram in january 1983 and a march 1982 b/w ad, both published in Musen to Jikken (MJ, Japan), the Ad-6 is an x-rare rarity - anyone ever seen this and/or used this crossover ?
A real Ad-6 with the ESOTEC tag, therefore early in the production run, is visible here (Gekko), and an Exceltronics version, sans ESOTEC tag here ; the Ad-5 version (made by "Chubuec") here.