I have three of the "Audiophile" series HD-212 (2x12) cabinets, two bought new and one bought used. They came out around 2001 and were discontinued and sold off on eBay for the
Casey Lee Ball Foundation. They're awesome, but ironically for anything Markbass, ungodly heavy. Shipping weight was 107lbs in the box. The original retail was $13-1400 per cab and the smaller HD-210 was $1000 and only marginally less heavy. A $1k+, 100 lb medium-sized bass cab that debuted in the middle of a recession had no chance, but they have the best midrange driver of any cabinet I've ever heard. I found a 4x10 online in Europe
here. IIRC, the 4x10 and 2x12 have the same external dimensions and the 4X10 was only available in the US by special order.
It's possible that Markbass' distribution deal through Guitar Center prohibits "Music Man" amps from US retail at GC and Sam Ash, GC's only nationwide competitor, already has Hartke as a house brand, so there is no avenue to get them to market (welcome to the monopoly marketplace!) That "BH-500" is probably just a cosmetically changed version of the
Music Man HD-500 amp that was sold with the cabinets. It's a nice amp, but was very overpriced when new, and not a lightweight either. I found a
review in German, complete with prices.