What is the lowest voltage acceptable ?

Furman should do just fine. I use older PS Audio regenerator that does voltage regulation too, in addition to other things. They are quite expensive even used and don't always work with powerful power amps, it depends on particular model.

The older PS Audio Power Plant Premier works well with my 60 watt/ch tube integrated amp. It worked well with my previous SS 80 watt/ch amp as well. Dynamics is not compromised. Everything is significantly better than without it or with older Furman voltage stabilizer.

I would not, however, recommend this particular model. There were many reported reliability issues. I got lucky, my unit has been trouble free since new, that's more than a decade. And PS Audio stopped repairing this model, I think.