Pream question

I was able to listen to VK-51SE and I wasn't impressed at all... The sound was absolutely dead and flat. Could it be because of the dying tubes? I'll try to persuade the seller to give it to me for a few days for the tests with my system. But right now LS-5 MKII sound really much better...

You won't hear the 51SE at its best unless you spend the "big dough" and get some NOS "DR" 6H30 tubes (talk to BAT). I tried some new production tubes which gave dreadful sound (and died within three months).
Nobody said the high-end is easy...

Can anyone tell me what caps should I use for 51SE as a replacement? Is it 0.1uf 400v or 1uf 400v by Jensen?

How do you know that the caps are the problem? I know that I recommended the Bat, but sometimes things don't work out. I was kind of surprised that it didn't sound good for you. The last thing I would recommend for you to do is put money into an expensive preamp that you've never heard sounding good. There's other choices, and not only that, you really don't know if there is anything wrong with it.

That said, if you want to look into Caps, I would contact Bat. They're known for offering Cap upgrades, and if you explain to them the problem, they may be able to give you some good advice. Bat does have a very good reputation. I don't think they would mislead you in any way just to sell a preamp.

Like talking to the wall...