Before you start tearing up a perfectly good Push Pull output transformer, bear in mind there are many sources for high quality Single Ended output transformers at reasonable prices. Some will even make a transformer to your specifications. Hopefully some of the information here will help you select and implement an output transformer.
Estimating the Specificatons of a Modified transformer
When you gap a transformer core, you greatly reduce the primary inductance. It is helpful to know what type of bass performance to expect from an output transformer before you begin. This will depend on the number of turns in the primary, on the dimensions of the core, and on the dc current flowing through the primary. Current depends on the design of the amp