any one box amps that come close to monoblocks?

Look for amps described as dual mono, preferably where they describe it as 'true dual mono' where it literally is 2 separate power amps all the way from beginning to end (separate transformers, everything)...in the case of some dual monos (like Gryphon Antileon) there are actually 2 power outlets, one for each channel so you need 2 powercords.

in your price range, second hand...Gryphon Antileon. if you google dual mono, there are a number. hope that is helpful.

Check out the M6PRX ($3500 list new), a dual mono 260w per channel solid state power amp by Musical Fidelity. There are several stellar professional reviews about the M6PRX on the Internet. There is a used M6PRX currently for sale on Audiogon for $2100. Musical Fidelity also makes a 500w per channel dual mono integrated amp, the M6 500i ($7000 list new). There's a used M6 500i currently for sale on Audiogon for $6000.

I have no connection with either seller.

I use the M6PRX (together with Musical Fidelity's M6PRE) to power my B&W 804 Diamonds and couldn't be happier with the results. The M6PRX is everything it's cracked up to be in the reviews.

In my experience there have been two,The Muse model 160,and the Plinius SA 100 MK III.The Acurus 8008BB was also very good!I'm sure there are many others.I believe all three of these amps were dual mono designs.When I owned them,I also had Sonic Frontiers SFM 160 monos,SFM 75 monos and Consonance Cyber 800 monos.I enjoyed the 3 SS stereoblocks as much or more.I still own/use the Muse amp.All of the amps had strengths and weaknesses.

That said,I have enjoyed many "one box" stereo amplifiers.While maybe not quite as covincing in terms of absolute soundstage,detail retrieval and authority,so many come so close!!!I've enjoyed offerings from Rogue,Cary,VTL,VAC and more.
my current favorite is a Raysonic SP-120 integrated.What joy it brings!

All of Bryston's amps are dual mono as well.