Looking for a well matched Preamp. Pls advise!

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.

get the best used conrad johnson tubed preamp meeting your budget - it will synergize with your adcom amp which can be strident and edgy... your kef speakers will thank you...  :)

Not sure anyone else bothered to catch this, but with your current TT/cart setup, your use of the Marantz is well-matched (if not over-matched). If at some point you upgrade your TT setup, THEN worry about a new preamp, if vinyl is your true concern.