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

Jeff -

Of the preamps you mention, I would go with the CAT. When I was shopping for the best preamp for my demo system, three brands stood out to me: CAT, Lamm, and Thor. I chose to become a Thor dealer, but think highly of the other two.

Kora is well worth considering. Lots of sound for the money.

One thing to consider for HT is that the pre-amp have a bi-pass or an input that has goes straight through with no gain for your main channels. In solid state there are quite a lot of pre amps that do this, but in tubes it's a bit rarer. If I'm not mistaken I believe the CAT does have this feature (it's also a great pre-amp), I'm not sure about the others. Of course, if you find a pre-amp you love that doesn't have this you can always have a particular volume setting that accomplishes this--but I've had a system set up this way and it didn't bother me at first, but after a while it got old setting the volume all the time.

Guys, thanks for your suggestions.

The Joule Electra LA-100 MKIII will blow all of the tube amps mentioned away. The VAC Standard would be a close 2nd.
Try the Blue Circle BC21 if you have a small budget.