I've never used KT120's in a Sunn circuit, but I have in a Reeves C225. I suppose whether or not the results are worth it or not depends on what results you are trying to achieve. I think the PT will probably handle the additional current draw, going to a SS rectifier will help of course.
But, what is the goal?
You won't get a 150 watt 200S, the power gain will likely be minimal. With the C225 circuit optimized for KT120's by the Reeves engineers, the power gain was 12%. Tonally it changed, the C225 has gobs of bottom end and as a result can seem a little mid "scooped", and the KT120's seemed to bring the mids up a little.
I didn't personally notice it too much, but another player using the KT120's in his Reeves thinks it's pretty pronounced.
I think if you designed a circuit and transformers around the KT120 you could have a beast of an amp, but just plugging them into a KT88/6550 amp doesn't have a dramatic effect or power gain. As I said, I haven't done it with a Sunn so that's just my guess. If you do it, be sure to post the results!
