If you want something vintage sounding but a better match for the HD then you'll really enjoy the emerald. They work excellently together.
+1 for that, Emerald does match well with quite a few contemporary pickups. However, like Troy said earlier, I too think that matched sets are just your best bet overall as they were made to match up. The HD is a very vintagey pickup like others have stated and playing with volume and tone knobs can easily take you into Mule territory if need be.
i do love how the neck sounds on the neck mule tho from the clips it sounds like thats the right neck humbucker. as for the bridge im not so sure. coz i want it to scream rock. so far its the nailbomb (which is chunky, doesnt cut through the mid range as much as id like) the holy diver (which sounds Rock, i wonder about pinch harmonics and riffage i really want the bridge to cut through but not tear your ears off like a fender amp, think intial cut into your face then a nice after effect.) the rebel yell (to me it sounds great but the sounding great thing covers alot of the playing so im not so sure on the dynamics of the thing as im all about the playing is in the fingers.)
i checked out the emerald it doesnt make me feel a connection to the sound.
for pure tone i love the mules but for a more in your face rock/metal sound on the bridge id like something like an amplified Mule with more hair around the edges.