I have not seen a lot of information on Buffet 6000 series flutes. While I still consider myself a beginner..maybe intermediate I do know construction and material qualities. I have a Jupiter Capitol edition flute and a Gemeinhardt model 3. The model 3 low end is not great, you have to work to get to it, it is older but in nice shape. The Jupiter plays the low end quite well, but becomes thin in the upper registers. I found a great price on a Buffet Crampon 6000 and took a chance. I can say the quality of this flute is better than either the Gemeinhardt or Jupiter. It feels very solid. Key noise is much quiter and it h as a solid low end. The upper registers plain clean and clear. I picked up the Buffet and can get to the 4th octave C fairly easily. The high notes come out clean and clear without over blowing.
I would definitely put the 6000 series up with the Yamaha's. Now maintenence on the long run... I will have to find out.