Out of all the headphone amps I have seen, none ever matched the Master 9 from Audio-GD. Looking at the Master 19 I can hardly imagine how and why a bigger/beefier amp would ever be needed.
The 19 is slimmed down version of the 9, while getting "95%" whatever that may mean. The 19 is B.I.G. and runs quite hot as it is a pure class A amp at 9 watt; balanced.
A relay controlled volume control makes sure there no form of channel imbalance at any volume. About that volume; from 0 to 99, with an L or H in front of it. The L(ow) gain uses the same logic as the H(igh) gain but uses a bit less range: meaning it doesn't degrade between H or L, just the range is different.
The frequency response is natural and neutral while the width is remarkable. Throw a focal headphone at it and it sounds like a normal headphone, but with the focal qualities. An HD800 might be overkill combined with the 19 but I never tried that combination.