New Parasound A21 VS Mark Levinson #23

Agreed....the Parasound Halo A21 is a great amp especially considering its modest retail price of $2500 new.
One of the best sounding amp in that price range.

Mark Levinson IMO makes outstanding gears, especially their preamps, but priced much higher than the Parasound.
I have never heard that Levinson #23 amp before so can’t really comment on that particular amp.

You can never go wrong with the Halo A21 amp IMO. It’s a steal at $2500 new for the kind of performance you get. It is a phenomenal sounding amp.
I think it will sound good on your B&W 803. Which B&W 803 do you have? 803 D2 or the latest 803 D3?

The flagship stereo power amp from Rotel called the RB-1590 will also sound great on your B&W 803. The Rotel RB-1590 retails for $3k and is huge massive and weighs in over 90 lbs. Rotel & B&W have great synergy together.
But in general IMO the Halo A21 is a better amp than the Rotel RB-1590. But you’ll never know.....the Rotel RB-1590 might pair better with your B&W 803. Who knows?

For your B&W 803 speakers if you’re willing to spend more I would look at the Classe Delta CA-2300 class AB stereo power amp. This amp, along with other Classe Delta series gears, were discontinued last year and nowaday it can be had in used after markets at discounted price. Its original retail price was $7k but since it’s been discontinued you can find a used one for around $4k ish. The Classe CA-2300 is a class AB amp and it will be an excellent match for your B&W 803 especially the latest B&W 803 D3 speakers. They will sound great together. Classe will pair really well with B&W. But again this CA-2300 amp is huge and very heavy weighs in well over 90 lbs.

In my dedicated home theater room my front speakers are the B&W 800 D3 driven by a pair of Classe Delta CAM-600 class AB monoblock amps and they sound terrific together and you couldn’t get any better than that. But of course performance wise and sonically the CAM-600 monoblock amps are step up from the CA-2300.

One thing you should note that the Parasound Halo A21 amp runs kind of hot so I wouldn't put it in a confined rack. The Levinson amps can generally run kinda warm too. However, both the Rotel RB-1590 amp and the Classe CA-2300 amp never run hot no matter how hard you drive them. They run cool the whole time. 

Oopps.....sorry didn’t see that the OP has stated the 803S. The older B&W S models before using the diamond tweeters totally have different sound profile than the newer models. I myself never liked the kevlar midrange drivers too as they distort and colored the sound and did not sound natural or organic to me. I never liked B&W until the newer D3 series with a new continuum midrange drivers instead of kevlars. B&W was never my go to choice for music anyway but they are great for HT and is one of my favorite HT speaker system. Except the newer 800 D3 models are quite good for music but still not my number 1 preference for music. 

In general, ML isn't in the same league as the Parasound Halo as ML is superior to the Parasound, but IMO the Halo A21 is still a great sounding amp especially considering its asking price points. I can't say that the ML23 is superior sounding than the Halo A21 as I haven't heard the ML23 so can't comment. The current new ML gears are very expensive and aren't really affordable to some people whereas the Parasound gears are much more affordable to most and Parasound Halo line makes quite spectacular gears especially for the money. 
But in general I wouldn't put Parasound in the same league as ML, especially when it comes to linestage preamps.