Check out their website where they discuss design features and have reviews posted for their previous models. I do not remember seeing any reviews on the newer models yet, although knowing the work of the designer, I assume they are as good or better than previous. Also do a search both here and on Audio Asylum's Amp/Preamp forum. I own the VK-500 and think it sounds quite good in my system. As a general comparison, the BAT amps use a simplified two gain stage zero negative feedback design with high quality matched N-Channel MOSFET circuits. Designers and opinions vary, with some others using negative feedback, and many using bipolar output devices. The all MOSFET design is usually said to sound smoother or more "tube like" (which probably means oriented towards a smooth musical midrange), but some say that comes at the expense of dynamics and bass definition, compared to designs using bipolar output devices. I would consider the VK-500 on the smoother and more laid back side of neutral, compared to an "up front" sound, but with at least adequate detail, dynamics and bass punch. This is where you need to have an idea of what your listening bias and priorities are. Also good system matching is important. The BAT designs are true dual mono, and in the case of the larger amps like the 500 and new 600, are like having two separate amps - they have two separate transformers, and even have two power cords. BAT is also big on providing a large power supply to improve dynamics, and offers upgrades of increased power storage via their BAT PAK, which is an optional block of capacitors. Opinions here vary, with many having their favorites such as Pass, McCormack, Parasound monos, etc.etc., but at that level most of the amps are at least good, and just sound different due to the different design goals and implementation. Speaker matching is important. Make sure the amp is appropriate for the speakers you are driving. Maybe some VK-200 owners can jump in here since that amp is closer to the new 250.
BAT Solid State Amps?
BAT tube amps have been discussed extensively here on Audiogon and in the audio press but I've seen very little discussion of their solid state products such as the VK250. In fact I know nothing about them.