Of course BMW PE is always a good choice with great sound, PnP installation, and OEM BMW part
For simple muffler swap (using a medium priced muffler) I would recommend either Rogue Engineering Ikon, or Remus as 1st choices, or you could go with Magnaflow (midpipe+muffler system available, or you can simply use the appropriate standalone MF muffler with some welding involved)
I have owned/experience with the PE, RE Ikon, & Remus - I would rate them as to sound quality in that order.
Since you're not looking for obnoxious sound, don't go with muffler delete, or the AA N52 exhaust (I have driven car with one, they obviously don't have the refined sound attributed by a muffler as it is basically muffler delete, and there is a very annoying drone right around 2500 rpm despite the fact they are using a heimholtz - and this is normal cruising rpm so you'll be hearing it constantly
)
As stated, there are sound clips for all these exhausts & more on YouTube for you to compare...