I have thought of this as well, i.e. to use a less powerful tube pre-amp to drive the 3XGT. Tried my cheapo Douk Audio P1 (sucks, avoid), considered one driven by 300B such as Feliks Audio Envy (too expensive) Willsenton 300B, or even Schiit Freya+ with upgraded GE 6SN7 matched tubes --but again, 3XGT is maybe not the best pairing for such a pre-amp, since any signal must be routed via the MUSES72320 /V6 volume control circuit, as far as I am aware there is no (approved) way to bypass that as there is for an amp that has a pre-in /poweramp function, such as does the Soloist 3XP. For the 3xP, I think in poweramp mode any of the tube-based pre-amps mentioned (except the Douk) probably would work fine.
An alternative would be rather to use the 3XGT is not a poweramp at the end of the chain but rather as a pre-amp into a tube-based poweramp /speaker amp. . My Chain is Roon from MBP --> Sonore Optical Rendu streamer-> Yggdrasil + OG-> Burson 3XGT -> Susvara headphones, so specifically I aim to add/improve soundstage with Susvaras. Options considered for 3XGT to act as a pre-amp: Willsenton R8 (likely not enough power), Willsenton 8800i (too heavy), Mytek Brooklyn Amp+ (not tube based, but reputedly has a nice, open stage and very clear). There are a lot of pre-cautions to take using a power/speaker amp for headphone listening, so caveat emptor.