I've got a Rocktron Metal Planet about 2 months ago and I love it, using it either as preamp in the fx return, or as (the perfect) booster in front of my DSL amp... It's a keeper for sure 😊
I also love a cheap Behringer TM300:
And of course I like my battery powered Blackstar Fly 3 amp...
I've found (after a lot of years' search) my holy grail tone using 600 euro analog equipment (a small tube combo & 2 pedals). For my use case, luxury is to buy a 1500 euro modeling gear in order to faithfully reproduce my tube rig's tone... 😉
Metallica released their new album in several...
I see the sig-x has a rectifier tube, so in fast transitions like what you describe
maybe the rectifier tube cannot keepup? In the manual says:"
LOSS OF POWER ON 40W MODE
1. Check rectifier tube (refer to qualified
service personnel)." My 2 cents & hope this helps...
So it is an urban legend, as it's totally subjective according to each one's personal taste (how we dial in our amp to our tone), not something specific for each tube amp, as it's being referring to, right?