This is a bit OT. As for treating the paper speaker cones, there is also a way to stiffen them and lower the resonance freaquency, resulting a warmer/richer/deeper sound. This can be done by PVA/PVAc glues which have to be diluted to water and then gently and sparingly applied with a soft brush to the cone.
I've tried this once, but I had too hard brush and also overapplied the glue. The speakers were Hitachi 3D80 fullranges. What happened was the cones deformed and when they dried, the sound was quite distorted. But still, I could hear a very clear difference on the "presence" of the sound and the low tones came through much better.
So, if someone has the courage to test things and a few spare speakers with thin sound, this may be a good way to get warmer sound out of them. I think the optimal way to apply the PVA would be to the underside of the cone, but it could be tricky.
Cheers