My ST120 was constructed in July of 2008, (one of the first three of Bob's ST120's). I have gone through 4 sets of KT88's over this time period. Brands include J/J, lasted 9 mo., Gold Lion, lasted 12 days, Elektroharmonics, lasted 8 mo., and finally Penta Labs KT88SC which lasted 13 mo.. Wish new issue KT88's were as durable a NOS.
At present the tube compliment includes: 3 CV4024, 1 J/J GZ34, and 4 Penta Labs KT88SC's (solid plate). I have concluded that for my money the Penta Labs have produced the widest frequency response, with no emphasis on any particular frequency range, and have an amazing sound stage. My speakers are ESS Heil AMT10B's I purchased new in 1976. Bob's amp has really made them perform. My ST120 was purchased as the basic kit with no capacitor upgrades and I have not regretted it for one moment.