Parasound or Anthem 5 channel amp

Parasound a51
Krell 575xd mono blocks for front mains

Those are two completely different sounding amps.  The Parasound amps are voiced slightly on the warm side and have a slightly dry sound in the upper mids/highs.

The Krell XD amps are very lush and smoothed over.  Good resolution, but definitely colored.  However, not warm/dry sounding like the Parasound.

The Anthem amps would likely match better with your Krell, but the Anthem would be more neutral and less colored/smoothed.

steven 0713, you might consider getting another Krell 575xd for your center so you have the same amplification across your front sound stage.  Then buy a 5 channel amp and have one channel idle.  Or buy 4 of the below Outlaw mono block amps if you have the electrical outlets available.

The side, rear and ceiling surround amps aren’t as critical as the fronts, so you could move amps around for those channels, and buy a 5 channel amp from Adcom or Outlaw Audio(both found on their internet direct website, for less money but good quality).

https://www.theaudiolegacy.com/products/gfa-5705-home-theater-amplifier
$3000, currently on sale for less, with free shipping.

https://outlawaudio.com/shop/index.php?id_product=45&rewrite=model-2220&controller=product
(buy 5 of these for a discount if you call them)
under $2000

Or you could get a Parasound 5 channel and have it drive your center, left and right, plus a surround pair, then move your krells to a surround pair.