Amp recommendation for RS6

Before you pick the naim, I think that you should try to demo it if you can. You mention that both amps are noted for being bright. You can argue that the naim may be bright and forward. I thought it sounded ok on naim speakers and a pair of BW lm1's on my computer. But when you put it on a better speaker like the one that have, it falls apart. I thought it did not have a clean sound. It is hard to explain, that's why I say listen to it first. The creek is just the opposite. It is more powerful and just as lively as the naim. The top end, though, is in a different league altogether. The reason that I went into the passive pre amp section in my first post was to stress this point. In my opinion, a good active line stage will cost a lot more than the type of components we are talking about here. I this case, the passive pre will work greatly in your advantage in that you are eliminating a large bottleneck in sound quality that an active line stage will create at this price point. The reason that I would not chose a tube amp in this setup is that your speakers, with a good british amp, will give you that propulsive sound that the british electronics are noted for. (although I am sure that tubes will be ok as will; just not my first choice. Hope that helps.

Thanks for the detailed explanation.
What interconnects would you recommend if I pair Naim to RS6?
Previously,with NAD C325,I used Supra EFF-ISL,Atlas Equator mkIII and Chord Crimson.
The best result was obtained with Supra cable which seemed more detailed and musical than the other ones.

You won't hear a big cable difference. Chord makes the naim cables. For the most part, I use Audioquest. They work well with just about anything as long as you stick with copper. At the time, I did an AB with the Jaguar and Cheetah using my Arcam 33 with the naim. I honestly could not hear any difference whatsoever between the two. I usually hear a big difference between them so I would not overspend. I can say, though, that I brought my CD player into my naim dealer to for a demo and I brought my AQ cables with me because I know them well, and the dealer was blown away by them. On the higher end naim gear, we did get some very good results. The dealer told me he was going to pick up the AQ line.

Tested RS6 with Naim Nait 5i and Audio Analogue Puccini SE.
Both amps acted very well but even the Naim was a little bit fatiguing in this combination,it brought much more openness,bass control,separation to RS6.
AA had a little bit more warmth ,the voices had much more body than Naim but the bass was somehow muddy and uncontrolled.Also lacked the dynamics needed to modern genres and rock.

I will also give another chance to NAD C355.If doesn`t work I will definitely go for Naim.

I used a CJ mf2250 with Silver 5i's...was an excellent combo.