pretty comprehensive list of your options:
for high-performing open options sub-$300: akg q701 on sale now for $199, akg k701 $265ish vs sennheiser hd h98 $190ish vs sennheiser hd 558 $130 vs beyer dt 990 pros (the 250ohm version is on sale at $150) vs dt880 (250ohm version on sale for $194) note: it appears the beyer pro vs premium have the same sound just different asthetics & cable I believe. phillips x1 at $300 (but it's price will probably drop down to $200 in the next few months) also, both the hd600 & hd650 are above your $300 price bracket. If you are willing to step up to the $400, I would highly recommend Hifiman's HE400.
My personal pick for open would be the akg q701.
for closed over-ears sub-$300: Mr. Speaker's modded Fostex trp-50s: Mad Dogs ($300), Sennheiser Momentum Over-Ear (on sale at $300), NAD Visio HP50 (newly released at $300), nonportable AKG K550 ($215), detachable cable version AKG K551 ($200), portable version AKG K545 ($200), beyer custom one pro $230 vs beter dt770 $170-$225 vs Shure SRH840 at $200. some budget closed options include: Sony MDR-1R (sub-$200; lowest I've seen $180), Logitech UE6000 (sub-$100; tigerdirect has em for $70ish or less I believe), ATH-M50 $100, Shure SRH440 $100, Sony MDR-V6 $86.
My personal pick for best sound quality would be the AKG K550 (if don't care about portability) or Momentums Over-Ears (portable) or Mad Dogs (based on what I've read - I haven't tried the mad dogs tho).