I wonder if not many of you have this same issue. I play the Stagg EUB, and very often the open E string is ringing as I play the upper three strings. I suspect this is true for other makes of EUB. I also play an acoustic double bass, and the problem is no where near as pronounce. My Stagg has steel flat wound strings, and the acoustic has nylon strings. I wonder if I put nylon on my EUB if this will solve the problem? Another bassist friend suggested I install a mild damper under the E string right next to the bridge. I tried that, and now the string doesn't sustain well at all! I would sure appreciate any suggestions you all may have. Thanks.
I use a sponge cut to press gently against the strings with my Stagg.
If the open E rings longer than you like after you have played it, you need to develop a technique to damp it. If there is a rest after it just damp it with one of your hands.
Otherwise use the following technique: Put the right thumb on the side of the fingerboard (at least if you play any higher string than the lowest one), pluck with the pointing finger parallel to the string and you will automatically damp any lower string when plucking the higher one with the rest of the pointing finger and flesh between pointing finger and thumb.
It's just a matter of plucking technique.