I don't dispute your choice of VMPS subs and the Marchand or even doubt your experience with the REL. What I do find rather perplexing is that both of the anomalies you describe (one note thump and collapsed soundstage), are the hallmarks of what a REL does NOT do in my experience. I have tried a number of subs in my years in this hobby and, IMHO, the RELs' incredible flexibilty of inputs/coarse/fine adjustments, high/low settings etc. make them emminently adjustable to fit your needs.
How high did you (or your brother in law) have it crossed over? Placement is crucial too as is patience in adjusting the controls. I have a pair of Velodyne ULD 15 MK-IIs that I formerly had in an HT set-up (and then tried with a set of Silverline SR-15s for audio only) -- not horrible -- but nowhere NEAR as musical and seamlessly integrated as my REL Stentor III with my main system. I have it crossed over at 23 hz though. BUT if I cross it over at even 40hz, I start approaching the normal "one note thump" you get with some subs.
Again, I do not question your knowledge as much as I wonder if something wasn't either wrong with the sub (e.g., defective) or if the room set-up was not optimized.
How high did you (or your brother in law) have it crossed over? Placement is crucial too as is patience in adjusting the controls. I have a pair of Velodyne ULD 15 MK-IIs that I formerly had in an HT set-up (and then tried with a set of Silverline SR-15s for audio only) -- not horrible -- but nowhere NEAR as musical and seamlessly integrated as my REL Stentor III with my main system. I have it crossed over at 23 hz though. BUT if I cross it over at even 40hz, I start approaching the normal "one note thump" you get with some subs.
Again, I do not question your knowledge as much as I wonder if something wasn't either wrong with the sub (e.g., defective) or if the room set-up was not optimized.