From 2007 until about 2011, and then again from 2014 until now, I've collected personal thoughts, RMAA and square wave measurements, and more, regarding various portable and desktop/rack MD units. The MZ-E55, which came out in 1998, and which does neither MDLP, nor bears an HD digital amp, is my favourite sounding. It was my favourite sounding hours after picking it up last week, and, with a few provisos, remains my favourite. It also tests well, though its amp shows serious weakness against modern Sony DAPs as well as iPods and the like.
The biggest proviso is amp noise. That is, it hisses on par with a Sony MZB100 - a long-time favourite of mine - all but precluding its use with sensitive earphones. Sure, worse hissing units exist, but sensitive earphones are de rigueur.
The good news is that it holds frequency response under load about as good as any top-end DAP or high-end desktop amplifier at any price. It keeps up solid stereo separation which congeals in the best possible place in order to maintain an airy and open sound field: the low frequencies.
Jitter, THD, and IMD are high, though likely none is audible. All in all, the MZ-E55 is probably the most stable of all MD units I've tested, and with the exception of hiss, about as good sounding a portable unit as I've heard. Also, Sony appear to have engineered extra bass even into zero settings for Megabass. I assume this is to overcome the crappy sound of earphones of the day. It's nice even today, and its Megabass, while sometimes duffy when set to 2/2, is soft-edged and complementary.
By percentages, deviation from RED BOOK 16-bit targets are as follows:
Noise level: ~95%
Dynamic range dB (A): ~92%
THD, %: ~260x over target
IMD + Noise, %: ~64x over target
Stereo crosstalk, dB: ~93%
Source: Sony MZ-E55 portable Minidisk player
ADC: Lynx Studio HILO LT-TB
Computer: 2012 27" iMac
Cables: 1,5m Hosa Pro 3,5mm stereo to dual 3-pin XLR (around 8$); bespoke y-split 2,5 TRRS to dual 3-pin XLR made by Musashi Sound Technology.
NL - no load
SM2 - Earsonics SM2
ES7 - Audio Technica ES7
DT880 - Beyerdynamic DT880/600
16-bit VOL (Full) @+0dB - all targets
16-bit summary (Full) @+0dB - NL
The MZ-E55 sounds good, looks like hell, and isn't so expensive today.