Buy several as you should be able 30 day return. It is fun to compare and before the 30 days is up you should have winner.
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You should also post on Head-Fi - lots of great info over there.
One recommendation you might not have thought of is a portable amp. Some of the best portables will drive Sennheisers very well and you could get double duty with your Westone 2 IEs. I find all IEMs benefit from amping the signal from your iPod, Zune, etc.
Look at Practical Devices XM6 (includes a DAC), Ray Samuels Audio SR-71A (amp only), or iBasso PB1, among alot of great choices.
Van Alstine has a very nice headphone amp included in his preamps. You can save some $$ and get one of the solid-state models, since the headphone amp stage is SS (even in his tube models). Then you'd have a great preamp as well. In the end you are spending closer to $1K (if you buy it new) but of course you get a preamp with the headphone amp.
This is a general consideration, dedicated headphone amps are a bit of a boutique item but it's pretty straightforward to combine it inside a preamp chassis.
If you want to spend "real" money, you can look at Woo Audio, they make lovely stuff but it will take you above $400. Unlike the Van Alstine, it's highly unlikely that you will be able to find a used one, unless you're both fast and patient (hard to combine).