Sony's Walkman DD series is well known for its sound quality among enthusiast thanks to its Disc Drive (DD) mechanism. The DD system has lower wow & flutter and sound distortion comparing to a belt driven system used in most cassette players. The RX-S41 has a similar Direct Drive system as the Sony DD line. Having listened to it I can confirm that it has similar qualities as the Sony DD and it simply sound amazing! Panasonic / National has another direct drive model the RX-S40 in the series. As far as I know, these are the only two DD portable cassette players Panasonic ever produced. So, yeah they are very rare!
There are couple of notable differences between the the RX-S40 and RX-S41. The RX-S40 is expandable and collapsible like the Sony Walkman WM-10, WM-20. It was an innovative way to make the device as small as cassette tape. However the RX-S40 has one major problem, it uses 2.5 mm jack instead of the commonly used 3.5 mm headphone jack. RX-41 corrected the problem by switching back to the 3.5 mm that is compatible with most headphones on the market.