Phono preamp

The Mani is decent but you’ll need interconnects. What cartridge are you using? If MM, you could look at tube phono amps. There is an Audio Research PH-3 for sale $750 on Agon.
If you wanted to change to a phono preamp instead, there's a Croft for $895. Both major improvements. You could them look at either an integrated or power amps down the road.

No! You can give it a try but I doubt it would be better than the phono section you have now!

Looks like you're back to square one?

I always benefited by finding out for myself......

question I have is the Sansui original? 

I'm asking because of the age and part start to drift and lose tolerances (Caps specially) some of the phono sections of those vintage receivers were actually quite good so you may want to consider some service work and replace some of the know failure items like power supply caps and the phono stage parts as they have closer tolerances. 

This may be the least expansive option and would freshen up your whole receiver. other then that the new parts in the Schitt phono may surprise you. 

I had my Sansui TU 717 tuner rebuilt ( little more extensive then what I had originally started with) total cost including muldorf caps in the key areas $400 ( Canadian so nock off 30%).