The GL KT88 have some nice qualities immediately but need time to burn in. I have only about 30 hours on mine but noticed they are punchy and have a nice sense of air about them. Mids are smooth but overall the presentation seems just a tad edgy if that makes sense. Been using some KT150s and have some Preferred Series KT88 on the way so really trying to get used to the KT series of tubes before they arrive. Since I listen to mainly dynamic music I feel the KT series provides better punch, slam and dynamics than the EL34.
JB I know exactly what you mean about instruments in space and right now I feel the GL KT88 is lacking that right now plus the fact the EL34 does draw you into the music more but lacks bass punch or lots of bass in general. I guess we have to keep putting hours on the GL until they are broken in and once they are I'm pretty sure we will know.