I recently add the Gradient SW-63 to my Quad ESL-63pro
I set them up as instruction, let Quad stand on the top of subwoofer, but I notice vibration sound between the two speakers, probably due to the subwoofer.
Any suggestion about this problem?

dingus uses a type of tape, or silly putty type stuff that might help.

elroy

Elroy is referring to blu-tac which i use to couple components to the shelves and stands they sit on, works great for affixing them securely together.

in this case i would separate the speaker from the sub and see if that cures the problem. even if you coupled them together, the sub is still going to be creating vibrations. i'm trying to get a mental picture of what you've got going here ... arent the Quads on their own stands?

The vibration did cease when Quad was removed even I turned the sound level of SW-63 to 2 o'clock postion; but the original design of SW-63 was served as the "stand" of Quad loudspeaker. I have no idea about placing 4 dipole loudspeaker in my listening room .
I'll try add some damping material between them this weekend.

The Gradient was indeed designed to serve as a stand for the ELS 63, besides its obvious sub-woofer duty.

You may want to use spikes under the ELS to avoid them picking vibrations from the subs.

Come to think of it, are you using spikes under the Gradients, too? That is essential.

if they were designed to go together i would think that any issues with vibrations would have been worked out in their design. are there instructions from the manufacturer that refer to this problem and how to solve it? failing that, Jorge's solution should solve the problem.