I don't know the range in question (frankly, I don't need to) but one defining feature of the 'thin wall' BBC cabinet is removable front and/or back or both front and back. If both the front panel and back panel are glued into position, regardless of the actual thickness of the panel walls, the box cannot be tuned. And tuning of the cabinet is critical to achieving the Harbeth sound. So, the first thing to look for is which panels are held in by screws. And yes, if you buy drive units for beer money from vast factories churning them out by the tens or hundreds or thousands each day then you cannot realistically expect sophisticated engineering. At the very best, you could expect a stable QC assuming that all the variables were well understood and controlled. But there again, even that can't be assumed.
I'm always incredulous that the consumer will pay $50,000 for a pair of admittedly novel or beautiful speakers using mass produced drive unit with a total rolled-up package cost to the manufacturer as low as $500 - or less. Possibly much less. To my mind that is complete madness - as foolish as buying a brand new, upper range BMW with an engine made in a Soviet factory during WW2. The car would indeed be impressive from the outside but a test run would reveal the performance was incapable of concealing the fundamental weakness of the engine.
Alan A. Shaw
Harbeth Audio UK