The KT-120's sound very, very nice in the GM-50.... No modifications required. They are a fantastic tube, and even more-so because of their most reasonable price in the current market.
Other excellent output tubes in the GM-50 are the original Tung-Sol 6550's and the straight sided Sylvania 6550's, there are certainly no shortage of options in this family !
Of course it's a matter of your system as to how any tube flavour tickles your fancy.... in comparison to the above mentioned tubes the KT-120's driven by Nos 6H30P-DR's display an overall fuller more relaxed character with great dynamics, presence and frequency extension at both extremes.
Experimenting with the input quad tubes can also take you anywhere you sonically please !
I've run the gamut from 6N1P, 6H23P, 6CG7 and many from the vast spectrum of E88CC and E188CC tubes....
I tell you what though, playing via my very sensitive horn speakers it sounds absolutely phenomenal for a relatively high power IDH PP amp when converted to run in Triode mode, without any feedback and running KT-120's.... Just HUUUUGE and sumptuously delicate but with an un-burstable mien ;-)
To do so however requires basic modifications, I have added switches to allow me to easily select Pentode/Triode and Feedback/No Feedback... amongst other things I've done to enhance its inherent qualities.
