Can you switch from a cloth surround to foam?

[Turbowatch2]

I'll try to get the 20mm foam surround, and then I'll update you with the results, or should I try getting the rubber one (h=18mm)? Is foam really softer than rubber and is it worth to compromise on durability?

The foam surround should last 10 years for sure. A rubber may last longer, but change it's hardness. High performance speaker do not live for ever like a cheap all paper cone in a vintage radio receiver.

 

[Radian]

Buy a ferrite ring magnet and stick it to the back of the magnet structure.
Opposing force north north or south south will lower Qts attracting force will up Qts.

In a German speaker diy magazine they made a speaker with the Visaton BG20 and by
attaching an additional ferrite Ring magnet on the back pole plate Qts was lowered from 0,45 to 0,37

 

[JMFahey]

Yes, that added magnet "pushes" original magnetic field forward and flux increases where it's needed: at the VC gap.
Not a lot but every bit matters.

 

[Vic1184]

Thank you all. I'm ordering the foam surround and I'll update you with the results.
I don't think adding a magnet is a solution as the existing magnet is huge, and the Qts is way too high to be lowered into what I need by just adding a magnet.