Do I even need a dedicated stereo preamp?

The Anthem MRX 540 gets good reviews as a 2 channle preamp, my thinking is your going to have to spend much more to get "Better" if not a side step/difference. 

Yeah I’m gonna take the other side of that.  AVRs are always big compromises as they shoehorn so many functions into one unit while designing to hit a price point, and this always comes at the expense of some of the more critical and usually pricier components like power supplies, capacitors, volume control, etc.  Then to put the delicate preamp signal through a morass of noise from all those circuits doesn’t exactly help matters.  Put it this way, in a $1900 AVR after including digital processing, video processing, streaming, a tuner, and amplification how much can be left to go toward the preamp?  Not a lot, so for all these reasons even a good budget preamp should  handily outperform the preamp section in most AVRs.  Anyway, food for thought FWIW, and best of luck adding a good stereo pre in the future. 

Do I even need a dedicated stereo preamp?

No. You don't.

Obviously you don’t NEED ONE, but would it be an improvement? It probably would be an improvement and could be a significant improvement. But you won’t know until you try. The suggestion above for the Schiit is a good one. It’s offered with IOS app remote control and has tube output, SS output and no gain output. If you get the remote option and the premium tubes is $1200 plus shipping and maybe tax. It has a return policy but I believe there is a fee. Your amp has balanced inputs but will accept single ended. I have Tekton DI’s and was actually recommended the Moab’s, but I went cheap. I have run mine with a $1500 dollar Rogue RP-1 with upgraded tubes to a cheap modded Atika 60 wpc ops amp. The DI’s are on Townsend podiums and it flipping rocks. Now I’m using a Rogue Hera balanced pre and a pair of 180 wpc Rogue M-180 Darks. And it flipping shakes the walls and sounds GREAT ! Tekton sells dealer direct and for what it is a remarkable speaker. There’s a huge fan base and a relatively large hater base. If I could afford a $15k pair of Tannoy’s or Fynes I’d by those. You could get a used SS pre that’s balanced like a Parasound or a Bryston , there are many. Or you could get a tube pre that’s not super syrupy like a Rogue. Look at Odyssey too. If your system actually improves with a pre , my GUESS is it will. The better the pre the bigger the reward. Decoupling those Tektons from the floor and proper placement will yield amazing results. Mine sound great with a pre and amp that cost $2000 total not including Tekefunken tubes and nice interconnects and power cables , but that stuff is reusable. With a $6K pre and $9k amps ( I was gifted the stuff ) it’s better , but not 7 times better. You have wine snobs and cigar snobs and stereo snobs. Don’t sweat it and enjoy the journey. Life is not the destination, it’s the journey and you’re right on track. Blessings and joy to you , Mike B. 

Need as in to make it functional, no.

Will it provide sonic improvements, almost always yes, and it grows in importance with better higher performing gear.  

Preamp for sure. Digital volume controls are almost universally bad at low volume. Only one that I found almost as good as my preamps on low volume was the Leedh volume algorithm on the Lumin streamers and DACs.

I have owned the Benchmark LA4 and the Holo Audio Serene at the same time. I now have the Benchmark HPA4 and the Schiit Audio Kara F. 

The HPA4/LA4 are super neutral, and I use it with a slightly warm CODA #16.

The Serene is a bit warmer than the HPA4/LA4 and likely a better match with the Puriffi amps you have.

The slightly warm Kara F is not as good as the Serene but cost $700 and I am enjoying the heck out of it with the Schiit Wotan amp and Magnepan Mini panels. I did not want to spend more on the preamp than the cost of the amp and Mini. It was a good move in retrospect. The HPA4 in place of the Kara improves the system but the Kara is not that far behind. This is the new version of the Kara.

BTW - I have only heard 1 preamp that I thought was better than the LA4 or HPA4 in my system. It was the new $33k Allnic Signature OTL/OCL I am getting the Allnic DAC that costs a lot but not the preamp because the HPA4 is pretty spectacular.

BTW2 - the $2k Wotan amp is a incredible value and preferred by me over the Class D amps I have owned in the past including GAN. It is a great overall package.

BTW3 - I recently repurchased the Schiit Mjolnir 3 (MJ3) headphone amp / preamp.  Previously It replaced the Serene and I liked it a lot for 2-channel (never heard it on headphones). After a while though I wanted a bit less warmth on the Mini. Today, I have some demo Focal Utopia headphones and the MJ3 is great with it. Keeping both.