"I connected the analogue output from a Sony X707ES CD player to a amplifier and the digital output to the Sony ES55 DAT recorder. Switched the DAT recorder as DA converter and also connected the analogue output to the amplifier.
With this setup I can't hear any difference between the CD-player output and the DAT recorder output."
Well,
sounds familiar to me

I have a Sony DAT DTC 670.
My parents a CD Player Sony X 33 ES
bought around the same time, I bought my DAT a year later, beginning 90s. These CD X33, DAT 670 and your CD X707 / Sony DAT 55 ES had all excellent DA-converter and analogue output. That's the money was worth it.
So, in early 90s, i recorded a lot CDs digitally from Sony to Sony.
Additionally I bought DSR (Digital Satellite Radio, with one packet of program 16 stations on one frequency = Tv channel) on cable in my location.)
Unfortunately they let it die.
The quality was too fine !!!
Still I enjoy the perfect analogue output of that DAT. My 90 tapes (later i bought 3 hours DDS PC-tapes, very cheap...) are still living, and I hope to record them (best of) digitally in my PC -> mpc, starting this winter.
Some time ago, I compared the digital and analogue output of a cheap Sony CDplayer.
Digital output in DAT, which converted analogue to amp.
And analogue output of that simple SonyCDplayer in that amp.
Guess what ?
The analogue output was not so brilliant at all...
IMHO there are listenable differences between different analogue outputs of players. If they are cheap and you compare to a good one.
btw, in that time I did a comparison DAT Longplay 12 bit 32 kHz vs. DAT originally recorded in 16 bit 44.1 kHz from CD.
Quite easy to hear the difference. Eg. the piano "beats" of "The Köln (Cologne) Concert" were easy to pick out.
Comparing amps:
Roughly 1.5 years ago i bought a Denon 1801 5 channel receiver. It is not the cheapest and had good test results.
But I felt, that it sounded too smoothly.
Lack of dynamics.
Studying more exactly the tests, I found, that the sound was described as "very well for classic, too, "chiffon_like" (hmm, hope you understand chiffon ? - means sound would be a kind of smooth.)
They described as advantage, and after that my sound impression was confirmed.
I had comparisons with little HK 620 amp (perfect sound for such a small and quite cheap amp), powerful Denon 1060 amp (very powerful, more expensive at that time as little HK 620, but HK was / is still a little bit clearer in sound) and that new denon avr 1801.
The Denon 1801 was clearly worse than my 2 other amps.
I sold it after 2 month at ebay, spent some money extra, and got the HK AVR 5000, which fantastic Logic7 switch i detected later during listening, previously I thought "all Surround trials in stereo music are crap", but Logic7/DPL2 changed my mind. I had good luck !