more lows than el34s. and i would say they're tighter than el34s as well. rather punchy, but not too much like kt88s.
with kt77s, the overall tone is a bit tighter and not as harsh. the highs and upper mids are very different than el34s. they're more glassy (in a good way). there's more clarity, and it translates to the ears and fingers as more aggression because of the punchiness and tightness in the higher frequencies.
i've never tried kt66s but kt77s are definately worth a shot.