I kind of disagree with this. hd 650's can be driven with cheaper amps, but more expensive amps pair better with them. This was the reason I didn't bother going for a 600 or 650, I didn't want to spend the additional money to really drive them.
OP, what kind of budget are you looking at exactly?
HE-400's have great bass for an open headphone. Going up in price the HE-500's are also great, and continuing to increase price LCD-2's as well.
Audio Technica ad900x is a nice balanced headphones. Not basshead level, but the bass is present.
Ultrasone pro 2900
There are many options at many price points
You are so incredibly off. "Cheap" amps drive Sennheisers perfectly fine power-wise, which is all you need. Power and a lowish output impedance and low noise.
Sennheisers are relatively easy to drive. Anyone who tells you different is trying to sell you something.
Oh, and those ortho's you are suggesting? Those might actually be a little difficult to drive, certainly more than any Sennheiser.
Edited by chrislangley4253 - 8/21/13 at 11:07pm