I happen to have the SRH 440 and listen to the type of music you listen to, except hip-hop.
I had the 440s for at least a year now. I got used to their sound and I actually prefer them for rock/metal/symphonic music. I just listened to Metallica's "... And Justice For All" album and the drum is very good, it's really punchy and controlled; it's not mushy and overwhelming like, let say, the AKG 240. For Dubstep though, you would probably be better with the other cans as they have more power in the lower frequencies.
But I'm getting tired of my 440s, and it's really becoming a deal breaker: Comfort. These cans are really not comfortable. First, the pads are way too hard and they just become harder with time. Second, the headband is close to not cushioned. What makes the headband really bad is that when you place the headphone on your head, the band flattens to the point where there's only a single point on top of your head that holds all of the headphone's weight. And this point is hard.
They are still very good sounding and this is why I'm trying to make them more comfortable. I tried the sock mod on the pads (found on this forum), but this doesn't fix the problem of hard pads. So I ordered the SRH 940 pads and will try them to see if it helps. I'm also experimenting with different ways of making the headband more comfortable... Now if you have to buy some extras stuff to make them comfortable, you should just go ahead and get the SRH 840. Better investment.
As for amp, I use mines straight out of my laptop and don't see the need for an amp. I tried a few, including custom built ones. The difference was too subtle to notice. In fact, the extra noise added by the amp just made it a downgrade. I believe an external DAC& combo would be a better suited upgrade for the 440. But this really depends on the hardware you have. I could simply be lucky to have a laptop able to drive them so well.