Where to invest the most in the "digital chain"?

You can’t recover lost DATA after the source , so I think of the streamer as a modern day "Transport"

While this is true, it is not a reason to spend more on a streamer. Every streamer, every ethernet switch, every cable, passes bit perfect data unless you choose an option to somehow manipulate it with DSP. Streamers are computers. Computers do not lose data.

If you are hearing differences it is not because any data was lost

Jitter is the bane of digital and reclocking the USB and Ethernet is the starting point.

. Data is sent in packets. The DAC does not use the clock from the USB or the Ethernet so reclocking the Ethernet and USB has absolutely no bearing on any jitter in the DAC.

If you are hearing differences it is not because of jitter from the source

I believe the DAC makes the biggest difference. I’m using a Musician Pegasus DAC, which uses .005 ladder resistor, the same accuracy as the Denefrips Pontus II and Terminator. I’m using a SMSL SD-9 streamer wireless which every type of connection to a DAC possible, including I2S to a Musician Pegasus DAC. I’ve owned a Topping D90 MQA DAC and while it sounds good, the Pegasus sounds analog to me and has me putting up the Topping DAC for sale. The Pegasus costs 1100 USD, and is worth every dime. The SMSL streamer is 399 and decodes MQA.

It sounds like you are at the starting levels of hifi. There aren't that many dacs for £1000. A lot of the models audiophiles recommend cost $3k or more. I have decided to be happy with what I have and wait a few years before upgrading. Then I may be able to find a nice, used dac for a good price.

If you want something now the Okto Dac8 has gotten great reviews and it has a streamer built in. Could be a step up for you.

In the interests of transparency I've spent the last several hours listening to a number of pieces of music that have passages of silence as well as single instruments and acoustic vocal tracks - with these on repeat I've slowly removed a number of components from the "digital path".

The end state I have landed on is that the Wi-Fi circuit in the Zen Stream is more than good enough for a low noise signal (hence removing the Cisco 2960, EE 8Switch and SotM iSO CAT7 and assorted high-end Ethernet cables).

The connection between the Zen Stream and the Zen DAC v2 is now just the supplied USB cable and an ifi iPurifer 3.

Removing the iPurifier and re-adding it yielded the most noticeable improvement in SQ with audible noise on an MQA Studio recording of Adagio for Strings without the USB filter, and near-perfect silence with it added in.

It is worth noting that I have recently upgraded my Wi-Fi router to an ultra-powerful TP-Link DECO XC80-5g which reaches every corner of my three-story house with rock-solid high-speed connectivity.

I have my eye on several potential upgrades to the Streamer and DAC in the near future, with a preference for a Streamer that supports Qobuz natively as well as Tidal with MQA decoding.

I appreciate all the responses, even the hostile ones as they are always a test of ones self restraint ;)

I have no filters or anything special "cleaning " the ethernet.   Just the N200 connected via SPDIF so yes, the streamer is the clock.    

The N200 with my DAC sounds great,  it's much better than the Vault.  Like I said before,  demo a really good streamer and it's unlikely you'll want to take it out.