I gave the PX51's a whirl and was thoroughly unimpressed. It's possible the Tango's have better EQ presets, but the bass on the TB's is off. Parts of the range were overemphasized, which ended up distorting the sound. TB's Dolby Headphone implementation also isn't as good as Astro's. The Tango's give better positional cues than the Golds, but not nearly as good as the semi-open A40's. The A50's are a closer competition, but still better.
Bottom line if that the Astro MixAmp is the best surround option on the market at the moment (this is pretty much undisputed--check out HeadFi, etc.), so they're the best choice if you're a competitive gamer and care about surround. This may change in a few months when Turtle Beach releases the Elite 800 and new tournament mixer. Both will use DTS Headphone:X instead of Dolby. DTS"s technology is very impressive, but it remains to be seen how well it will work when working off a converted Dolby signal, as none of the consoles support HP:X natively.
I'd agree that Astro's aren't worth $150-200 more than the Tangos, but they're available refurbished from Amazon and have been as low as $140 for A40's and $165 for A50's in the last month. They're worth THAT premium. The only build issue I'm aware of with Astro's is the slight right cup hiss when the sound gate is active on the A50's, which does drive some people bonkers.
You're wrong about the Gold's being stereo-only as they have a Sony-proprietary virtual surround implementation. It's not as good as any Dolby implementation, but it's there and its serviceable. For someone who doesn't care about surround, the Golds are more comfortable (as has been noted here several times, the Tango's run large--the Gold's will be a better fit for MOST people) and deliver stereo quality that's almost as good as the Tangos. And they've routinely $50 cheaper than the flash price on the Tangos.