Welcome to the forum. I have one of Bob's ST120's that I built over four years ago. Great amp. As to the KT88 power tubes, I have tried Gold Lion, Elektroharmonics, JJ, and Penta Labs. All of the tubes made in eastern Europe had quality problems/failures in less than a year. Granted when they did work their sound was very good, (I think that Bob's amps are really what make the tubes sound their best). Gold Lion failed with two weeks, JJ made almost a year. The tubes from Penta Labs are their KT88SC version with solid plates and are made in China by Shuguang. These tubes have lasted the longest. They are going on 3 years now and they sound great. I have come to the conclusion that it is the luck of the draw regarding tubes now days. The old saying "don't buy a car made on a Monday or a Friday" surely would hold true in this situation if we could choose. Being that the ST70 uses lower voltages you may have better longevity with the KT88's. Hope you have many enjoyable years of listening, I know that I have.