BAT SS amps - Newer VK-250 or VK-500 w. BP?

I have always been curious about the BAT "house sound" and had the opportunity this weekend to demo a VK-250 (w. BAT-PAK (BP)) in my system (RWA SB3 direct to amp to Escalante Fremonts). This combo is detailed, balanced, quiet, refined and slightly laid back. The bass is perhaps the best I have heard from these speakers in terms of both quantity and quality, bettering even the new Nuforce Ref 9SE v2.

My questions to BAT owners who have heard the VK-250 are:
1. How does the VK-250 compare to the VK-500 or VK-500 w. BP?
2. How does the VK-250 compare with the VK-220 sonically?
3. How much (%) improvement does the BAT-PAK make? (my dealer also has an upgraded VK-500 (revision D board), but no BAT-PAK)?

The obvious downside to the VK-250 is its current cost relative to the VK-220 or the older VK-500. The VK-250 and VK-220 would certainly fit in my audio rack more easily than the VK-500.

Thanks for your advice!