Sound quality impact - preamp vs power amp?

Equal. However, most modern amps have enough gain that they can be driven by the source components (assuming decently sensitive speakers), so a good amp only needs a passive preamp with quality input switching and volume control. So start with a good amp.

Yes, everything gets connected to the preamp but all signals eventually make their way to the speakers. The speakers are the heart of the system and have the greatest influence, especially if they’re decently efficient. 

The acceptable phrase is silk purse out of a sows ear. Either way its bollocks. A better preamp will be a better preamp and an improvement regardless. Speaker amp problems are speaker problems. Trying to solve the problem of a hard to drive speaker with an amp is a fool's errand. 

People asking about separates while pretending to care about cost always make me smile. If you really do care about the cost/performance ratio, in other words value, the answer is almost always an integrated. With integrateds like the Raven Reflection you have to go pretty far up the high end food chain to be looking at separates- and then you're talking $50k not the (relatively) measly $11k the integrated costs.

But the audio community has guys so spellbound buying ever more stuff, and separates do help you buy a lot more stuff. You can get mono-blocks, bi-amp, and lots and lots of power cords and interconnects. 

So a line stage preamp is the frosting and an amp is the cake, which can be layered in so many ways.  Both vitally important.