Replace C28 with... ?

It might be helpful if you explained why you are replacing equipment.  Are you trying to achieve a specific sonic goal?

Not happy with my current SAE A202 amp - lacks staging depth, imaging is sometimes confused, it's a little thin, and not quite as musical as I'd hope.  Though it drives the DQ10s well enough, I think there's room for improvement, hence identifying it as the weak link in the system and replacing it with the 4BST. See my post here:
https://forum.audiogon.com/discussions/another-which-amp-question

I listen to a wide variety of source material, mostly Jazz, Classical, traditional Rock and some pop, so I'm more or less all over the map.

Here are a few I have on the radar, in no specific order of preference, but need input from you guys who have more experience based on my equipment since I don't know what characteristics or advantages each may have over my C28:

Cary SLP-50
Threshold FET1
Krell KSL
BAT VK3
Conrad Johnson PV14L
McCormack Deluxe Line Drive
Tortuga LDRx

Other 'non-mainstream' suggestions have been:
Bottlehead
Prometheus TVC Reference

I looked at your other thread. Did you get the Bryston yet?

It’ll be here tomorrow. I’ll probably be sitting on the steps waiting for the UPS truck to pull up.

Another question to add to the list, is whether I ought to be looking for a Preamp with balanced outputs, since the amp supports that.

Why don't you wait and hook the C28 up? You just might like what you have now. Don't put the cart before horse!