Can anyone help me to decide which pre-amp is better ?

Lots of good suggestion here. You might also want to consider one of the new Sonic Frontiers Special Edition units, particularly Line 2 or Line 3. Very high value for the money, terrific sound, and good ergonimics with remote control.

The vk-5i and vk-50(SE) do not have optional phono boards. You must buy an outboard unit such as bat's p5 or p10.
The vk-3i and vk-30(SE) do.

The vk-5i has a fairly different sonics to the 50SE. They both have the bat 'house sound' which is a gloriously warm midrange, feathery well defined high notes, and excellent bass. Also, to paraphrase J Scull, the music has a 'breathing' quality. The 50SE is more transparent and revealing. The 5i has a little more of a 'glow' or vintage tube sound to it. I do run NOS tubes. One of the unique characteristics of bat tube equipment is that it produces extended high notes, yet remains warm and listenable. SS preamps can be a little 'too accurate' when playing high notes.

I own the CAT sig.Recently did an AB with the Hovland and the VK 50SE and the Herron. The Hovland is real pretty, and has by far the best WAF factor. The CAT has far sharper claws than any of the above. IMHO waaay better bottom end than the Hovland, faaaar more dynamic than the Herron and loads less grain than the BAT.
The Phono stage on the CAT is also something to consider.

I vote for the Kora Eclipse pre-amp. As far as I know this is the only tube pre-amp on the market that offers:
1. fully remote operation (everything from source, volume, and on/off) 2. both MM and MC inputs
3. balanced and single ended op 4. Headphone outputs
5. processor circuit (remote controlled)
6. true high end sound that is high class B or true Class A.
thanks, Rich M.

You missed the best one.. Joule Electra LA100.

Rob