These links offer a good discussion. Sag is considered more for guitar amps.
https://www.kcanostubes.com/content/rectifier-comparison-and-interchangeability-chart
https://www.aikenamps.com/index.php/what-is-sag
You’d want less sag, but it’s relative. If you can hold on for a few hours, I’ll share what tubes I’ve used with my Mogwai SE. I’m a happy camper.