Piper Jaffray analyst Troy Jensen says his firm's 3D printing survey for Q3 indicates system demand "remained challenged" in the quarter. The below plan responses "significantly" outnumbered resellers indicating above plan, Jensen tells investors in a research note. The September quarter survey had the lowest percentage of above plan responses in the history of Piper's survey, the analyst points out. Jensen advises caution on 3D printing stocks into the Q3 earnings season. The analyst sees limited downside for Stratasys (SSYS) shares and more downside for 3D Systems (DDD) given its "internal issues and premium valuation."