No doubt the Woo's are wonderful, but at a price, of course. I've found great synergy with my HD600's in a Peter Millet Mini-Max, which (if you are into DIY) can be built as a kit for about $260. Or purchased used or from various builders over at head-fi.net
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.
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