Doggie, instead of putting more effort/money into this amp I would consider getting a new one. For example, if you don't mind DIY, a Bottlehead amp. I built an inexpensive Crack BH that uses 12au7 and 5998 tubes. With a simple adaptor you can use a 6SN7 instead of the 12au7. However, it does depend on your headphones since the Crack is an OTL design and only drives high impedance HPs. For Sennheisers it's great.
Another plus for the Bottlehead is the ability to customize it (caps etc.).
Good luck!