No problem, I saw your question and thought maybe you were headed straight to the top in equipment.
If your list is based on your preferences (Metal first, Rap last) based on your list, I am assuming you do not have an amp.
with $300,
(1) I would recommend picking up an amp...even a simple E9, sound cards tend not to drive higher headphones, and even my Grado SR60's which unamped are great! But a simple USB amp ($30...seriously!) helps quite a bit...so thats a thought...depending on your source, you may want a DAC as well, some are all in one unit...just keep that in mind.
(2) With Metal being a priority I would look at open cans, they tend to have more highs and mids than closed, just based on the design. I had a pair of the K701's which are similiar to the Q's, and I will say I liked them alot for metal, I once told Arainach they were like Grados but better. Big soundstage, and very 'accurate' sound. I didn't feel that the can was alot of "fun" but I loved the detail. I never did listen to it unamped, so I cannot comment if that would be a way to go.
I have not heard any other pair you are comparing it to...but I would suggest you to check out the HD598 might be a good fit too based on the reviews I have read. It too, seems to be drive okay without an amp.
(3) I would make a suggestion to pick up a pair of Grado's, even the SR60i (used around $40-50, new around $60/70) they are the only pair that managed to survive into my 'now I have kids' stage, and for metal and rock, they are very good, and do not need an amp to get what you want.