You need to avoid saturation at all costs, and most mains transformers are designed by penny pinching misers to have minimum copper and iron, and will be hurting pretty badly at 265v
Only way to get more turns (and more iron) will be to use a much higher voltage rated transformer.
Not by a bit, but by a lot, like 400v or more.
That will have an amazingly beneficial effect on flux density at "only" 265v
The other issue is your amplifier. That will be much more efficient if run at a higher voltage and a lower current, if that is possible.