As soon as I started to read this thread, I didn't think the cable length was an issue. I have the same pair of subs. I use a 6 ft cable between sub and Rhythmic amp. Mine work fine. I love the bass in my system. I do not play loud. But the bass is low, rich, and elastic. I have a pair of 2x12 OB and a REL T/9x in the back corner. 3 subs total. The volume is set fairly low on all 3.
Your issue about no documentation is spot on. The fact that none of these kits have a PDF file of best build practices and other important build info is defiantly a miss. The standard answer is to look through this forum. Ask your question there. Sure, that is good for unique questions, but the common stuff should be captured in a document. And the answers you get in the forum can be wrong, and ofter are. Not from Danny, but from the hundreds of others posting here. This thread is an example.
Creating an instruction document for each kit would take too much time..... Nonsense.
Most of the doc could be a cut and paste from these forums, as Q&A happen, for the common stuff. Danny would oversee this, if not do it himself to be sure doc is correct. These sub kits have been in existence for over 10 years I think. Imagine how good the build doc would be by now. What's the full price of the kits for these 2 subs, with flat packs, and no simple doc?
Danny does great work! We just need a doc to answer common questions as we build, or before we buy.