The TDA1543 is the economy version of the TDA1541 and does not use the bitdecoupling caps.
Sound of the TDA1541AS1 is considered better. The TDA1541AS<B>2</B> is impossible to get, at a fair price.
Sometimes you just get lucky. A few months ago, someone posted on the Asylum (either tweaks or digital) that he ordered 1541 pieces from one of the biggies (maybe Parts Express or MCM, I don't recall right now, even though I ordered and got a few) and got S1s, for real cheap. I'd guess that, as an "obsolete" part, they aren't even being evaluated and graded anymore at surplus or wholesale operations. - Pat