[Mondial]

Ok then. if the transformer has dual secondaries, you can keep the negative feedback and it should work fine, regardless of the step up ratio.

I just saw your further comments:

The original input transformer was a step DOWN transformer while this replacement is a step UP transformer.  A local radio guy here says that using a step UP transformer will increase the gain on the output tubes and tend to introduce distortion.  He would NOT remove the negative feedback circuit because that tends to reduce the gain which is needed even more since the replaced transformer is a step up transformer.

The step up transformer will increase the gain of the output stage, but you can just turn down the volume to compensate. It will not introduce distortion unless you try to play it too loud. If the replacement transformer has dual secondaries, you can keep the negative feedback. The feedback will not work properly if the replacement is a single center tapped winding.

Also, the local radio guy here (with about 40 years experience but not on this exact set) suggests maybe putting shunt resistors across the secondary windings to bring the measured resistance down from 1.8K ohms to something like 200 -400 ohms. Would the transformer (with a primary of about 715 ohms) then function as a step down transformer OR would I just be screwing things up even more?

Shunt resistors are not a good idea. You will just be loading down the output without changing the turns ratio to step down, which will cause distortion and clipping in the driver stage.