Amp vs Preamp

Depends how "juicy" your preamp is. 

An amplifier can be colorful, but it also has an interaction with the speaker load, so hard to drive speakers may sound very different depending on amplifier.

Swapping out very neutral performing preamps may do little for you, but going from a neutral to juicy/colorful preamp will seem like a big step.

Personal experience tells me there are so many factors that addressing the weakest link makes the most difference. Narrowed to preamp/amp again the weakest of the two being replaced gives the most benefit. The following can be a litmus test as to what factor one might see as the most important/critical in a system. For each of you I ask, what piece of equipment would you be most resistant to replace and what piece would you least like to see replaced?

For me a recently acquired preamp has made the most difference. Once the change was made the shortcomings thought apparent in my amp largely disappeared and the strengths of the amp became more apparent.

Experiences had of Pre -Power Amp combo's, strongly suggest from an end sound assessment  that a Power Amp' that stands out when matched with one Pre Amp' Type,  can be experienced to appear mediocre, a shadow of what it can be, if the correct match of Pre Amp' is overlooked.

Whether the Power Amp' of a different type is able to lift a Pre Amp's performance, that has not matched well to certain Power Amp's in experiences I have had, is an unknown to me. 

My experiences are that the Pre' is the enabler to that very special end sound.

No preamp still can work with PVC or TVC, but the volume range will be a lot smaller than with preamp.

It seems from the comments that the process is a bit of trial and error vis a vis matching Preamps and Power amps.  I suppose it might be easier to stay within the same lineage or company when trying to match the components.  Or at least using that as a starting point.