I can't speak for the newer breeds like the Ax-FX, Helix or profiling amps like the Kemper (Steve?), but the older modellers like the Line 6 stuff could only do so much. The reason has to do with what real tube amps do in that last 1% (the "magic" if you will - sag, feel, whatever you want to call it). There simply wasn't enough processing power to waste a ton replicating that last 1%. That said, I think it can be done now, just not sure if it has!