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Yes, these are parts I have unused at the moment leftover from a recent upgrade.  Thanks for the input.  I will keep the 3070ti for a secondary gaming pc in the future.

There's really no reason to go for a 3070ti, unless you plan on live transcoding many high bitrate streams simultaneously. Even then it would be a questionable choice. 1070 will be more than enough for most use cases.

If you aren't heavy on encoding with it then if it's only goal is plex it's overkill. I have a Ryzen 5 1600af running a server for TrueNAS that also serves Plex, with a few cores left over for a DNS, and a DMZ. Use the 1070 and almost feels a waste of a 3700x but if it's what you have around doing nothing may as well. Would recommend trying Jellyfin also. Plex for me has issues finding the right boxart sometimes and hates some x265 encodes as well but I've not managed to figure out why.

The only reason to go with the 3070ti would be if you wanted to bare metal game on the HTPC. Otherwise go with the 1070

Are these parts you already have? Because this is extreme overkill for a HTPC, and when I say overkill, I mean killing an ant with a nuke.
 
How many users and what source quality (namely resolution) and what resolution do you need to transcode down to? If you don’t need to transcode you don’t need a GPU at all. 8 GB of RAM would be plenty, and the CPU is way overkill, an Intel i3 from 10 years ago would be plenty. 

I have aquired a FiiO E10K USB DAC and Headphone Amplifier, I just don't have anything to plug into it.  My monitor is a MSI Optix MAG341cq  ultrawide 1440p 100hz that I have overclocked to 120hz.

Thanks for the info! 
 

Thank you, I guess that is what I was trying to figure out.

Well, my first, as in it was my Dad's when I was little, was a killer 8k TRS 80 Color Computer.  We had that when the Atari was new.
 
My first build was a 486 40mhz but was missing the math co-processor.  I think I spent $200 on 4 mb of RAM, more on a sound blaster soundcard and a killer 9600 baud modem.