Is there anything out there that intergrates with HA and controls the volume (line in) level of an amplifier?
Yes but how depends on your goal
You basically need preamp with wifi control
Technically you can get an Onkyo TX-NR626, for example, and use that as input to an amp. since it is wifi controllable you will get audio input (to amp) control. Onkyo receivers integrate directly with HA (no third party app or server needed, Local LAN needed only)
Chromecast allows volume control. I use these in Whole Home Audio setups along with amp. This way amp is set at Desired Max Level then volume adjustment is through CC. I figure this will last until next best thing and next best thing will likely allow volume control. I believe audio control in HA is possible but I forget
Some TVs have variable audio out. this too can be used and many TV integrate with HA.
There may be some devices available that do this as well. I also have some ATLAS amps that themselves have direct LAN controllable.
Whats the project/goal?
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Just need to add a volume control to the line in of my amp. Will have a look into your suggestions 
Thank you!
I have a need for this as well but wasn’t able to find anything cheap enough to justify this fairly simple feature. I just ordered two cheap (<$10) I2C controlled digital potentiometers based on the PT2259 chip. It will likely require some work to make it talk to an ESP based board or a Raspberry. I’ll update this thread if I can make it happen.