KT90's have a lot more power. I don't have KT66's (I do have EL34's) and I do have a quad of KT90's and they do have power. More power that my 6550's or KT88's.
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Dhcod, can your amp handle the new KT-120 tube? The KT-120 is a drop in for a number of ARC amps that previously used the 6550 Winged C. You should check with the amp manufacturer before even attempting it. There a number of posts that warn about problems with power trannies, aside from general electronic compatibility issues.
If you do a search on the Forum, you will see a lot of discussion about the KT-120. A number of members who switched over report that their amps have crisper better defined bass and more airy extended highs. Others speak to greater power output, robust build quality and hoped for extended tube life. Please note that the KT-120 draws a significant amount of current, so check with the factory first.
Cool! Yes, the KT-120s DO generate more heat, and heat is not a friend of electronics. You may want to buy a fan to keep the amp's "innards" cool. And, be sure there is plenty of air flow too. If you try the KT-120s, please report back with your impressions. The tubes need at least 25++ hours to break in. Make sure bias is set properly.