I have exactly the same impression for Stax SR507, I feel it's sound a bit etch and have a small soundstage.
But it stays incredible headphone with a detailed, liquid, fast, clear tone. I have a SRM007t amp.
I want to upgrade to HD800 balanced with headamp gsx or beta 22 or dna stratus. This kind of combo keep the positive aspect of my previous combo : liquidity, transient decay, clarity, overall balance ? I will have more fuller tone with better imagining and soundstage ?