The best sound ever I heard on minidisc portables was on my ex. MZ-R900.
If you no need NetMD and want to use the unit only for listening - MZ-R900 is your choice.
I have compared a lot of units with the playback of the same minidisc and the same track recorded via optical in SP (true SP).
So it means that i compared the difference of the influence of electronics on these devices.
I must note that R900 sounds better than my Hi-MD RH910 on the same track.
The best sound on the non-MDLP units was at MZ-R70.
I must note that i haven`t listened all the minidisc portables, so may be I`m wrong. But i have listened a lot of them.