Do I even need a dedicated stereo preamp?

The A6 DAC will hold you back... great as streamer only, but not the DAC.  Find a used McIntosh D150 DAC/Preamp and you will see radical improvement for about $1,600.

A quick AI search said the 540 digitizes all analog inputs so the output from your Cyan is unnecessarily going through additional conversion back to digital and then back again to analog, which along with all the other AVR compromises I mentioned earlier this is yet another reason why a good stereo pre will likely make a significant difference.  I was also gonna mention the Freya but it doesn’t have a real HT bypass although there is an easy workaround for that.  Anyway…

Hey OP

I had the opportunity to own some of your gear. I had the Benchmark LA4 combined with 2 x AHB2. my speakers however where ATCSCM 11. I used it for about 1 year and I figured out, AND this is me, it's just too "clinical" for my taste. 

so, I ventured into Tube Preamps as I did not want to lose the AHB2. I sold the LA4 and got the Schilit Freya+ and that is where my audio Journey started

as you have stated, you like True to source, detailed retrieving and BIG soundstage. and I believe your speakers (Tekton with a lot of speakers and sensitive) driven by your Purify amp will just do that. I am not fond of using AVR as preamps. period. Been there and done that. I owned Denon and Marantz AVR and all these AVR seems to affect the overall impact of the music. the impact may be profound or subtle, nevertheless it has an impact

I also own the Eversolo DMPA6 Gen 2, coming from a Gen 1. Yep, they do sound very very good.

My suggestions are the following options. 1) due to your tight budget, Use the DMP A6 as your digital source and Pre-amp. directly connect this to your Purify amp. Listen to it for a few weeks. then return it back to the original configuration. you will hear a change. then choose. 2) second option, go get a Schiit audio Preamp called Freya +. I also own this and is my standby Pre-amp. it has options you will like, and the cost is not too steep. it will allow you to choose direct to connect OR use Tubes to flavor the music. 

The La4 is a great Preamp, kind of expensive and may be too much for your current gear, so I suggest that you look into just using the Eversolo as your preamp and as 2nd option, use Schiit gear,

GOOD LUCK. your gear sounds great as it is

The A6 DAC will hold you back... great as streamer only, but not the DAC.

I do only use it as a streamer. I'm using the holo audio cyan 2 as the dac. 

I was also gonna mention the Freya but it doesn’t have a real HT bypass although there is an easy workaround for that.

Whats the workaround?