Tubes for VTL ST-150

Yes. See this thread.

So if I read this correctly, a KT88 would be a workable replacement, and other 6550's besides Electroharmonix would also be oK?

 Gold Lion KT88 appears to be the consensus of owners. Upscale has 6550EH.

My used Cayin A-88T came with 6550’s.I switched from 6550’s to KT88’s, prefer the KT88’s.

Note: Bias. IF you change tubes, it is recommended to adjust the bias. My amp sounded better with KT88’s, without adjusting bias.

My MK1 version of that amp has internal bias adjusters, amps with exterior bias adjusters and even bias meters are much easier.

Your amp features says individual bias adjustments for each tube.

https://www.vtl.com/products/power-amplifiers/stereo-amplifiers/st-150/

I had mine adjusted, sounded the same, before and after. (it is possible it had originally been biased for KT88’s, and seller just put 6550’s in without adjusting bias). Part of why KT88’s sounded better.