Is upgrading pre worth it?

A huge difference in my opinion the preamp makes a bigger  impact on sound quality than an amplifier. And going from a receiver to even a modest well designed tube preamp should really be a quantum leap forward in sound quality.

kingdombuildingcon it's no problem mixing ss and tubes assuming there are no impedance mismatches or anything but I think, and others here too, that a tubed preamp is the way to go. In fact many do the reverse of what you propose and pair a tubed preamp with a ss amp. I would say unless you have some compelling reason to get a ss preamp stick with tubes.

Congrats and happy listening and please do let us know how it all sounds!