Torroids are smaller and lighter, which is the main reason they’re used is SS gear. Using a torroid as a mains transformer on a tube amp is okay, but it has a wider footprint, so it’s a matter of allocating real estate that usually takes priority when deciding to use one. Torroids are not used as output transformers for one main reason: any imbalance in tube plate current or gain will alter the a.c. operating point and create lots of distortion. Also, it would have to be bigger in size to guard against saturation at low frequencies. But if every tube was identical and wore out exactly the same, then torroids would be far better output transformers.
Toroidal transformer on tube amp
Why are toroids so popular in solid-state amplifiers but so uncommon in tube amplifiers?
They are said to send less EMI and interference to nearby components, among other benefits.
Is there a reason why solid-state designs are "so good" but tube amplifiers aren't?
Why not use them as output transformers?
I've never used EI transformers and have always relied on toroidals.
But, in any case, I've never built a tube amplifier.