Do I even need a dedicated stereo preamp?

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.

Yes you do! Preamps aren't just volume attenuators, they also do impedance matching. I've tried it both ways.

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.

I like that it has both tube and SS, but unfortunately I only have android devices in the house. They say they are working on it (no idea when that was stated). 

Decoupling those Tektons from the floor and proper placement will yield amazing results.

A few months after I purchased my Tekton's I put AUVA 100 feet on them. Big improvement on my suspended wood floors. I also spent hours in REW taking measurement after measurement. I moved them around probably over 20 times. Once I found the most optimal place for them within aesthetic reasons and of course WAF, I used a laser measure to ensure they are symmetrical in my room. I have first reflection panels on the left wall and a heavy curtain on the right wall along with a big couch and a large rug. Room treatment is nearly perfect. I think as diffuser on the rear wall would be nice, but the panels, curtain and rug make a huge difference. 

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.

That's good to hear. That price is more manageable.