Mikirob - Are you looking for a sound quality or more power than a 6550/KT88 or KT90? I ask because most people think they will get more power by dropping a KT120 in place of a KT88. I asked that question because my thoughts were without having higher plate Voltage and Current you won't get any power.
http://www.audioasylum.com/cgi/vt.mpl?f=tubes&m=248736
That being said with the specs of the KT120 you should be able to drive it harder than a KT88 before arcing over. It is supposed to be one 'tough' tube. Personally I haven't tried the KT120 yet but am looking to upgrade my Octave V70SE to a V110 (made for both 6550/KT88 and KT120's). Plate voltage on the V70SE is 540V and the V110 is 540V for 6550/KT88 mode and 620V in KT120 mode.
http://www.audioasylum.com/cgi/vt.mpl?f=tubes&m=248736
That being said with the specs of the KT120 you should be able to drive it harder than a KT88 before arcing over. It is supposed to be one 'tough' tube. Personally I haven't tried the KT120 yet but am looking to upgrade my Octave V70SE to a V110 (made for both 6550/KT88 and KT120's). Plate voltage on the V70SE is 540V and the V110 is 540V for 6550/KT88 mode and 620V in KT120 mode.