While comparing amps to each other, drive your headphones very loud, something like in 100db-105db range. Differences between amps become very accessible around that level, and once you hear the difference, you can hear it while listening around 85db too. (Remind you, 85db is 4 times less loud than 105db.)
And as for headphones, you can never know how they sound without listening. Try as much headphones as you can.
Disclaimer: I am fully biased, since I have a KGSS for sale!
With that said, I feel the Gilmore amps are head and shoulders above the mid and entry level stax amps, such as the 252.