If you don't feel anything from your usual dose of sub anymore then you will most likely be able to mix cns depressants without too much worry. Only you know yourself if you can handle any type of combo, but this one you're asking about isn't that bad of a one if you keep Phenibut dose under 1.5 grams. Try out a gram first, and work your way up from there. 1 gram is usually the threshold "feeling something" dose for me, while a half a gram can just leave me without anxiety but nothing more in the effects department. Just watch when you go above 2 grams, because without a tolerance the high from that can be pretty uncomfortable in the side effects department and the hangover.
