With the stock tubes it's pretty good. Can't remember the exact tubes used, if you want to know I can look them up. Just enough tube body and warmth is added to a give the amp a different character from my thx 887, which sounds rather dry in comparison.How does it fare on stock tubes with the HEKS?
However, with the Riverstone GE tubes the clarirty and detail go up a notch, bringing it into great territory. Because the amp is only 100 and the tube upgrade is only 23, I'd say just upgrade.