Looking for a well matched Preamp. Pls advise!

This Bryston BP-6 should be a considerable improvement over your Marantz (I used to own one) — more open/expanded 3D soundstage, and you’ll likely hear tons of detail from your KEFs that was previously being masked.  It’s like straight wire with gain and is completely silent and transparent, so if you’re looking for a preamp to add warmth this will not do that.  It’s got plenty of inputs and dual preamp outs to handle your sub.  No phono preamp, but most phono preamps in integrated amps aren’t very good so if/when the time comes you’re likely better off buying a separate phono pre anyway as it’s a pretty important component.  Best of luck  

https://tmraudio.com/fresh-arrivals/bryston-bp-6-stereo-preamplifier-17/

If budget ender  $1500. The Schiit Freya preamp with 6 sn7  tubes.$1199

I don't think Adcom preamps sound very good. I had a GTP-400 and it was meh.

You might consider Erhard's preamps: https://www.erhard-audio.com

How is your sub connected now?  Both the Schiit products have an XLR and two RCA outputs.  My preamp is another brand, but I have both of my subs connected to XLR outputs and the two monoblocs to the RCA outputs.  Works fine, so it should not be an issue IF your sub is self-powered.  Another option would be to look at the preamps from Audio by Van Alstine, although they are strictly single ended (RCA).  Their three models are highly rated and run from about $1000 to $2200, although adding a phono circuit would be another $379.

https://avahifi.com/collections/preamplifiers

I have a similar system ( i have the Meta though) and i use a vintage pre amp: the Onkyo P-301. They are inexpressive and  the sound is great.  They are sturdy, have many futures that modern pre amp don’t They are also fun to use and look at...you can find them at Ebay.