Heed makes a superb head amp for the price. Uses discrete dual mono output and a large toroidal power supply, not a wal wart power supply. I bought mine for $500 and it will drive your Sennheisers with authority.
Headphone amp for Sennheiser HD 600/650?
What are the most synergistic headphone amps for use with either the HD 600 or 650?