Yeah the price difference is not small. However, the Audio-GD amp can hang with the Pass. But - I do like the Pass better over all. I also like Audio-GD speaker amps way better than their headphone amps. There is a very noticable difference in sound and I prefer the Power amps. After I heard the Master 3 with the HE-6, I went out and got the Master 10.
If you only had Master 10 money I say go for it - it will not disappoint you. You will be very happy.
As for a comparo.. I'll start with what I like the most about Master 10..... The sexy ass mids. Female vocals sound so good with this amp. Now What I like about the Pass amp. Better bass - the Pass kicks like a mule. The Master 10 has full bass just a little more bloom compared to the Pass. The Pass amp has more air and the soundstage is more open. The Master 10 is more of an intimate amp, more on the warm side, less air, smaller stage. The Master 10 has buttery smooth treble. The Pass amp has really good extended treble and still smooth - so the Pass amp spits out more detail - but the Master 10 is detailed as well just as much so as the Pass. Both are just effortless.