The SUMO trademark appears to have been cancelled in 2009. Here is the record from the U.S. Trademark and Patent Office --
http://tsdr.uspto.gov/#caseNumber=73714069&caseType=SERIAL_NO&searchType=statusSearch
The company was bought by Califone International in the 1980s. Here is an interview that mentions it, originally published in 1988.
https://www.stereophile.com/interviews/rich_may_of_sumo_an_audio_dynasty/index.html
The former president of Califone, Michael Custer, is now CEO at Chatsworth Products, an IT services company.
http://tsdr.uspto.gov/#caseNumber=73714069&caseType=SERIAL_NO&searchType=statusSearch
The company was bought by Califone International in the 1980s. Here is an interview that mentions it, originally published in 1988.
https://www.stereophile.com/interviews/rich_may_of_sumo_an_audio_dynasty/index.html
The former president of Califone, Michael Custer, is now CEO at Chatsworth Products, an IT services company.