hello my friend,
For KT KT120 and 150 I don't think they will be compatible, because the heater current needed is higher than kt88 (1.9A vs 1.6A), and it will put more effort in the power transformer.
that can result in more heat generated as well as transformer core saturation.
Unless
@Andykong says the transformer has enough margin to take it, I would'nt try.
For KT 66, they should be compatible,because heater current is lower (1.3A) but, they have only a 25w dissipation rating in comparison to the 42w rating of kt 88.
depending on the operating voltages and Bias , they could run too hot if you let the rear switch in Kt88 mode. You can try with the switch on the EL34 setting to be safe, and see how that sounds.
In the official documentation, kt66 is stated to be compatible in el34mode :
KT66 is a beautifully linear tube, results should be good. I haven't tried it yet, but I would like to hear from you if you do try them !
for power tubes i tried kt88, el34, 6ca7,6l6gc, kt77
personnaly, I like 6l6gc best for my utopias, and el34 best for my planar hifiman arya
in general i find kt88 to be hifi , almost solid state like with great bass and details,lacks a bit in refinement, and el34 more romantic and refined with emphasize on mids and rounder sound, good soundstage.
kt77 are like el 34 but with more highs and lows, but a bit unrefined.
6l6gc are very lush with good balance and nice details and nice soundstage
6ca7 where a bright and very detailled, refined, and pleasant but only lasted about 2 months ( don't know if they were faulty to begin with or if the amp run them too hot )
if you try kt66, I will wait for your impressions !