These decoders are - well - not easy to find these days - TdP decoder is pretty much unobtanium these days.
For what is worth - the mono performace out of, I think both of my Leak IIs, is spot on, and simple RCA splitter gives you faux-stereo (well, rather duplicated mono) performance where the bass is tight and articulate.
The sound of the mono is just missing that last part of the stereo FM broadcast.
There is a guy in the US who makes a custom decoders for the tube - valve FM tuners, and says that the sound coming out of these tuners is pretty much what you get out of the decoder, although my small objection would be that the tuner itself got to be a good one to begin with.