New Parasound A21 VS Mark Levinson #23

The Mark Levinson has a very unique sound to the midrange.  It's hard to describe if you have never heard it.  It's sort of like a very thick sounding midrange with unique and different overtones.  Some people love it, others do not.  You really have to hear it to understand.
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The Parasound A21 is slightly on the warm side, but it is a more conventional sounding amplifier.  It's not in that "different sound" category like the Levinson.  For my own opinion and preference, I think the A21 sounds more natural and open than the Levinson.  I am in the camp that likes the Parasound and doesn't really like the Levinson.  It's all a personal preference.

Having personal experience with Parasound A21, I will say that the Parasound sonic signature is definitely on the warm side of things.  It is a very good amp and has a very refined midrange sound, but still on the warm side. 

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Parasound uses excellent parts and components and they have an excellent overall design.  I have found that the one thing that holds back the Parasound is how many fuses it has.  Parasound likes to fuse the heck out of their amplifiers.  The A21 has 5 fuses total.

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You can make the Parasound A21 a world class amp by just investing a little more into fuse upgrade:

1 x Hi-Fi Tuning Supreme LARGE 15A for the back panel main fuse.
4 x Furutech LARGE 10A for the internal post-transformer fuses.
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When using Furutech for all 5 fuses, the sound was still excellent (much better than stock), but was still too warm for me (I lost resolution and attack).  The single Hi-Fi Tuning Supreme silver fuse boosted the speed just enough so that I had excellent attack / resolution / detail.

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However, in your case, with the older 803S that uses the aluminum tweeters and the older kevlar midrange breakup issue, you might want to try out all Furutech first.  400-500 hours of burn-in required for Furutech before you should make a decision.   

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The older kevlar midrange had an issue with break-up which caused distortion in the sound and could come across as too bright/harsh in some systems.  A slightly warmer amp can mask this problem as well as the bright/breakup of the aluminum dome tweeter.

Which B&W 803 do you have? 803 D2 or the latest 803 D3?

ask17 - I'm not sure who you're asking the question to.  Your 803S is the last of the original Nautilaus series before B&W moved to the diamond tweeter.  If I remember right, the "S" series had a better aluminum tweeter where B&W was able to raise the resonance breakup to something like 28khz.  I used to have the B&W D2 series and now have the D3 series.  I'm driving them with fuse-upgraded JC1 and A21 amplifiers.

By the way, there are a couple "brand-new-in-box" Parasound A21s on ebay for $1999.  One in silver and one in black.  I actually bought mine from the same seller a few months back.