between late 80's and early 90's.
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The DQ-20 were released in late 1987 or early 88. Here's a link to a Stereo Review article (in PDF). The DQ-20i made small improvements with a different midrange and slightly different crossover. There's an active group of Dahlquist fans over at Yahoo Groups if you want/need more info. The DQ-20 and 20i are killer speakers. Best, Mark
Prices on these fluctuate wildly. I've seen restored DQ-10s go for $500-1000. Ones that need help can be found for $200. DQ-20's are often listed between $500-700.
According to the Stereo Review article mentioned above, the 10's were still being produced in 1988. I have no idea when they stopped making them. You might want to pose this question over at the Yahoo Dahlquist group: