any one box amps that come close to monoblocks?

Have to agree with Phaelon . And would add that a stereo amp at the same price will trounce a pair of mono's every time .

Let's see... LAMM M2.2 are $25k ? or so. Are there stereo amps that will clearly be better in most applications?
But if we are talking used pieces, things might be different. Say, Rowland 8Ti that was, I think, $6k here a few months ago. Try to better it with monoblocks.
So it is not so linear, it depends including on particular piece at particular price.

Pricing may be a touch higher than your budget (with discount who knows?) but The Gryphon Diablo sits nicely in the category you aspire to !!