I think the polarity that you mention regarding the VK-51SE revolves around the 6H30 tube. Some people love it for it's dynamic's and power, certainly more dynamic than other tube designs. Others don't like it because of a sense that it sacrifices the midrange palability that make tubes popular. I owned a 51SE for awile, and I would not characterize it as dark, the warm side of neutral maybe, but not dark.
I believe the polarity is based solely on the 6H30 tube, which seems to be gaining popularity now that ARC and CJ are using in their top preamps too. For me, it is a very tough call. I did love the dynamics/transparency of the 6H30 tubes, but I did not enjoy the more sterile midrange presentation.
That's my take anyway. From those that I've talked to that hated the VK-51SE, most blamed the 6H30 tube itself. They laugh at the 'Supertube' reference and say give me my old 6DJ8/12AX7/6SN7 preamp and let me roll with NOS tubes. Since you won't find any Mullard/Telefunken/Amperex/Siemen's 6H30's, it's hard to say if it's the tube design itself or the lack of good NOS tubes.
If you had no choice but to run your Line 3SE with Sovtek 6922's, you would have some idea of the midrange sterility that I speak of. Of course the benefit of this is the price of NOS tubes is getting pretty outrageous, especially if you need 8-10 of them to fill your preamp....OUCH!!!
Just my $.02
John