Why are Lundahl output transformers used (abbreviation: OPT)?

Lundahl output transformers, unlike standard output transformers, have a high-quality dual-coil C-core design made of grain-oriented silicon iron strip-wound core material (VM111) and are suitable for both single-ended and push-pull tube amplifiers. Different core air gaps and configurable primary and secondary impedances allow flexible impedance matching while making maximum use of the windings. This significantly improves coupling and therefore bandwidth and impulse response quality. Optional amorphous cores are available, which can offer sonic advantages, but have around 30% lower signal handling capability.