i own WP6s and medium powered amps just don't make them sing. the WP5s are an even tougher load than the WP6. when i went from levinson 33h to 33 it made a world of difference. the WP6 will still sound good with medium power. i tried the red rose amp 1, which has twice the power of the nagra vpa, and it was very dynamicaly limited. the red rose sounded good, but did not allow the WP6 to really strut it's stuff like the 33s or the halcro dm58.
i now have atma-sphere ma2 mk2.2 connected to the WP6. this amp works great with the WP6; there is no percieved dynamic limitations. it doesn't quite have the effortlessness of the 33 or the halcro, but it is much better than the red rose or even the 33h as far as dynamics. also extended in the bass and treble. the MA2 is easily the best amp i have heard.(period)
the atma-sphere MA1 might work on the WP but i have not tried it. it would be in your price range.
the MA2 is more $ than your upper limit. i would personally suggest that you at least demo a high powered amp with the watt/puppys before you settle for medium power so you will at least know what you are missing.