Krell FPB 600 = to what new amp today.

I don't think you will find anything with the bass slam of that Krell era - not even the current Krell's nor d'Agostino's.

Having used the humble (only 300 watts+/ch @ 8 ohms) Bryston 4B3 in my current system, I didn’t think it sounded "thin" in any way (Used full range w/ Mac C22 preamp -probably your speaker wire). Also, The bass I got from twin 6.5 inch woofers (B&W 804D3) bordered on "intimidating" (speakers were bi-amped with Bryston on the bottom). Bryston cubed series are fine amps.

Morning,
I have a very similar system with the Matrix 800’s, Krell FPB 600 and a Beard P100 for the Mids/tweeters. I auditioned some Classe mono blocks (can’t remember the model) and was underwhelmed. 
I then started to a hunt for the original “partners” to the 800’s and eventually found a total of 5 x MPA810 Monoblock amplifiers. Due to the age and that only two of them powered on they are all being refurbished at the moment. And therefore I have yet to place them in my system.
The only other amps I would like to audition would be from PS Audio but being in Northern Ireland I am yet to find a supplier who will allow me to have them on loan.
Your room set up looks similar to mine but I have the staircase (open) behind mine as I found the sound better than against the wall.
If you do find anything that comes close to the Krell do let us know as I would be interested as recapping the Krell will need to happen at some point and there maybe a modern alternative.
Iain
BTW I use a Krell 250a as my “back up” and it gives a similar sound just not the punch of the FPB.