Whether you add mass to the speaker, or clamp its sides, or change the platform/stand the speaker is sitting on, or change the way the speaker couples to the floor, you are altering the tuning. Whether this is good or bad really is a matter of taste, so experimentation is the only way to find out what would be beneficial.
There are a number of speakers that, by design, have thin walls that vibrate quite a bit. They are designed to rapidly dissipate vibrational energy instead of storing and releasing energy more slowly (which is what thick, heavy walls tend to do). Examples would be Spendor classic speakers and Harbeth speakers. I recently heard the Harbeth 40.2 and I thought that this is a pretty good speaker, so that technique does work.