I followed up the idea in this thread - what happens when a good headphone amp is used as preamp in a speaker system. I have actually not tried this before in my main system. Or not since way back. So I brought Audiotailor Jade OTL down to the main listening room, disconnected the preamp, and connected Jade instead. The result was quite good, but not on the level of my (much more costly) preamp (Einstein The tube mk2). It would have been a sensation if it sounded better.
So my experience is that inserting a good tube preamp into the amplification chain depends on the quality of the rest of the gain stages in the chain. For example, the Jade easily beats small s-state amps in active speakers, from my testing so far (low to mid price). But if the other gain stages are in fact of comparable or higher quality than the Jade, the effect can be zero, or reversed.