@lono24 Sorry to hear about those reliability issues, I get that. From the ULD to the DD Plus my only issue was a faulty IR receiver on a Plus. They repaired it for free along with a new remote and well out of warrantee.
For Velodyne it wasn't noted until the early 80's release of the DD series. Sadly, it seems you may have overlooked the About Room Placement and Tip sections on pages 22-23 of the User's Manual describing the "first step" which would have eliminated the trial and error and vastly improved your DD's presentation. https://velodyneacoustics.com/pdf/digitaldrive/DDManual.pdf
You were just one model away. The completely redesigned DD Plus provided foolproof twenty minute Auto/Manual EQ room optimization for dummies like me. In the User's Manual on page 25, again the first step out of the box, the "crawl test". Manual adjustments were amazingly simplified with the unreviewed Frequency Response and Parameters Screen function of pages 10-11. http://www.velodyneacoustics.com/.webloc
Regardless of the vast differences in our rooms and systems and my own in home comparison with a $9K Studio III, I'm certain had you properly positioned the DD's and used low frequency test tones played through your mains and adjusted the DD-18's accordingly you would have returned the sub-bass speaker.
I'm a hack hobbyist and its clear what led to your subwoofer dissatisfaction, again, I get it. In my experience as a Bassist, due to their rolled off -6dB I respectfully disagree with your final statement. As with most things in this adventure your personal satisfaction is what matters in the end. All the best. m
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