matthewpiano:
3) Buying used kit is a lottery. You can't audition it and you can't send it back if you don't like it. You don't know anything about whether it is performing to spec unless you spend further cash having it serviced, and then there is the risk that the amp will end up sounding different.
4) Sideways moves are a complete waste of time, whether dealing with vintage or modern kit
5) Once you reach a certain level of performace, such as that offered by the Marantz 6003s or my NAD kit, a massive investment is required to make anything like worthwhile improvements.
Agreed with 4, and to some extent 3, in as much as it is a lottery unless you do some research ahead of buying anything (but in truth that applies across the board, new or used).
Some sellers will take returns, esp if business sellers and you buy direct. But ultimately, it comes down to knowing what kind of sound you like and the equipment that will best deliver it, or come closest to it. There isn't I should add, one set-up that has delivered the kind of sound the live experience delivers - many have come close (my current set-up probably closest) - but that's another angle.
5, yep, sideways moves in hifi, best avoided. Couldn't agree more; what you in one area, you lose in another. Sometimes there's no gain at all, just more of the same.