I know it's been several months since this post and I'm butting in but there are so few of us who still own The vernerable Vortex Screen that I am gonna butt in.
The Brochure says 87db and the surrounds are indeed of rubber.
In my experience 50 watts will limit the Screens to moderate volume levels. Especially on music with loud peaks where the Jolida will run out of gas. I started with a highly rated Threshold 100 wpc Stasis amp but wound up loving the results I got with a 200 wpc Sunfire. My system sounds better than I would have imagined. I was happy with my 100 watt Threshold until the Sunfire came along. Doubling the power made a surprising difference considering that it only offers a 3db improvement in volume. It makes me wonder what 400 watts would sound like. Especially if it were a tube amp. (Like I could afford one)
It also helps to explain why Bob Carver is making 300 watt tube amps and McIntosh it's 2000 watt transistor amps. Tube power is different from solid state but I would think you need a lot more headroom that your Jolida is providing.
Having said all this, I once hooked up a nice old Marantz SM-6 Esotec amp. In class A it puts out 30 wpc and it sounded pretty good. It wouldn't give live performance volume levels but it was still surprisingly clean up to a point. The bottom line though is still about power and a 50 watt amp needs a very efficient speaker to rock the house. The Screens might be the best "affordable" loudspeaker ever made and I would recommend you find a amp within your budget that can do it justice. In thirty years of chasing the best sound I can afford, and in the process spending a whole lot more and hearing whole a lot less, I have never had better sounding system. Von Schweikert does make a speaker rated at 99db just for low powered tube systems but they will set you back a lot more than you paid for the screens.
