The 8820 soundstage is wider but the 8920 ss is deeper, the 8820 sounds a bit more clear and balance, 8920 sounds darker, and smoother, more analog like. I prefer the 8920 to the 8820.Btw, i'm from Vn to :)))
So you mean if two amp have the same frequency respond, they will sound the same? According to measurement of Tyll http://www.innerfidelity.com/images/AllAmpGraphs.pdf every amp have identical frequency respond, except some have minor roll off at 20hz cause by the input capacitor, which is small different. I should just buy the fiio e6 and save bunch of money.