KT120 had been stock in Rogue amps for many years now. You can run the KT120 in these amps at a higher bias (40 - 45mA, or even 50mA if you’re bold) than KT88 (35mA). In operation, they should be fairly close with more headroom (plate dissipation) on the KT120. For both the Rogue Apollos and VAC 200iQ’s I’ve rolled tubes in, Gold Lion KT88 vs. KT120 is just a matter of preference. KT88’s are more refined, slightly sweeter midrange; KT120 have more fullness and bass presence, overall a very voluptuous and "powerful" sound -- this is why Rogue switched to them (also they suffered through many faulty batches of Electro Harmonix KT88 in the 2000s). But the KT120’s upper mids can be a little rougher and drier in comparison. I didn’t even notice that for the first few years, but over time it wore on me a bit. That’s why I switched to Gold Lion KT88 in the Apollos, which aren’t naturally sweet like the VAC amps.
KT150 is a very different tube; they were safe to run in 200iQs but sounded awful. That’s where I buy the argument that an amp needs to designed *specifically* for that tube. But KT120 vs. KT88? Nah, they’re close enough for many models with adjustable bias and auto bias schemes. Roll and have fun, if you’ve got ’em.
Currently running Mullard KT88 in my 200iQ’s, and these tubes are excellent as well -- they will definitely sound great in the Rogue amps. But the KT120 are really excellent in those VACs. The VAC’s midrange covers for the KT120. I hope production files in again.