ratchet freak - I've got the soul sand (25k or so) but only 16 wither skulls so far. That's why I was so annoyed about the wither skeletons escaping...I've had the spawner running for maybe 5 minutes, and I've had wither skeletons escape twice. I guess I could use the auto-spawner (which would hopefully avoid the compressed mobs), but that means I'd need a better supply of mob essence, which is just a hassle I hate dealing with (mostly because it means I need two mob farms, since grinders dont give all the player only drops.)
you can use diamond spike on your mob farm. This will give you player only drops and drop XP. Then just collect the XP w/what ever way you want and when you need to convert to mob essence just use a XP shower onto a sewer and this will convert it.