DAC upgrade or Amplifier

With all things being equal, the dac/preamp will generally have more of an effect on the sonic signature than an amplifier will.  However, they are not equal in your situation.

The Cambridge Audio stuff is generally know to be very detailed/transparent.  You could upgrade to a newer/different DAC, but if you want to keep working with your DAC magic, I would get an external linear power supply.  Go to ebay and search for "teradak dacmagic".  It comes with a cord that plugs directly into your DACMagic.  You can get one for about $180 shipped.  This will definitely improve the overall sound quality and soundstage of your DAC.

Your GFP-750 preamp is a nice choice.  There are ways to improve it:

http://bigskyaudiomt.com/ADCOM_GFP750.php

https://hoppesbrain.com/2017/03/11/adcom-gfp-750-upgrade/

If you have it upgraded, see if they can replace all the electrolytic capacitors as well.  With the age on this, the caps will have dried out.

Probably your weakest link is the Adcom amp.  My experience with the older Adcom amps is that they are warm sounding, but in a mushy/messy way.  They end up smearing the sound somewhat and I actually think they introduce distortion.  Definitely not a good option if you want a deep/wide 3D soundstage.  I would try selling it on ebay.  You can get a really nice high-resolution amplifier that would be superior, such as the following choices:

Parasound A23

Emotiva XPA-1L monoblocks (with fuse upgrades)

Rotel?

others... depending on your budget

Guys, let's not get carried away with the chase for MQA.  I'm sure the bitrate and quality of MQA is good, but it is not the entire story.  You have to make sure your DAC and analog stages are good as well.  If you can ensure the DAC/analog are up to par, then MQA is a great solution for providing better resolution for streaming audio.

I know everyone is talking about room layout, acoustics, sources and tube preamps and such.  That's great.  I would like to re-iterate the limitation of the amp.  Like blindjim stated, "everything matters".  I 100% agree with this.

I used to own an Adcom 545 amp (100 watts per channel).  It was one of the first steps in my audiophile journey.  I did do some mods to it (blackgate caps, upgraded A/C to D/C diodes, hard-soldering all plug contacts).  The mods definitely improved the sound.  However, in the end, I determined that the amp just did not have the sound quality.  When I bought a Crown CTS 600 (Class AB amp), the sound quality went up drastically.  The Crown was just so much cleaner with much more detail and "separation of instruments".  It really showed that the Adcom was just messy/dirty sounding in comparison. 

While the Crown CTS 600 was an absolute amazing item for the money (less than $200 on the used market), there are definitely amps that are much better.  When I upgraded to Emotiva XPA-1 monoblocks, they showed that the Crown was definitely on the low end of sound quality.  The resolution, strength, punch of the Emotiva was a lot better than the Crown.

The Parasound A23/A21 amps will be much more refined than the Emotiva, which is why I put them on the list above.  The A21 is an awesome amp, but definitely much more expensive at around $1600-1800 used.  The A23 will be around $700-800.

Looking at the internal architecture, the Sony HAP-Z1ES is a significant/huge upgrade from your Cambridge DAC Magic.  However at $1259 used / $2k new, it uses up pretty much your entire upgrade budget.  It leaves nothing left (in your budget) for amplifier upgrade.  The assumption is that the Sony does use a digital volume (which means you can connect direct to an amp).  The digital volumes works by converting the 16/24 bit audio data to 32 bit and then reduce the bit height of the waveform before it hits the DAC chip.  There is controversy on whether a true analog preamp is better than this or not.  Personally, I think I would rather have full bit resolution at the DAC/output stages and run it through a true analog resistor-ladder volume control.  A high end passive preamp could be a solution (like the Khozmo or even some others that have a capacitor/transformer output), but passives do have their own challenges as well.

I have not heard any preamp less than about $10K that can do a decent job.

Steve's quote may be absolutely true from his point of view, but I caution the need to spend $10K plus on a preamp because everything is relative.  You can definitely get a better preamp with more transparency/resolution, but everything is relative to what you can actually work with (both budget and practicality).