I just got a Nucleus and I have a legacy equipment not Roon Ready and I am not sure how to get the best audio from them

[Bill_Janssen (Wigwam wool socks now on asymmetrical isolation feet!)]

Yes, you could do that. Whether or not you get a superior sound is another matter, depending on a lot of things.

I use USB to a Topping D70 DAC which goes straight into my power amp, for instance. I use the analog side of the DAC as a preamp.

There is an issue with digitally controlled preamps. Bad code or resets or bugs might cause them to turn on with full volume and ruin your speakers. Many recommend putting a passive preamp in between the DAC and the power amp. See this thread:

Use a Raspberry Pi endpoint without a preamp?

I have to ship my VTL tube preamp off for repair, which motivates me to use Roon more than just to a Chromecast boombox. I assume I can get a Raspberry Pi + DAC combination, install some Roon endpont software on it, and plug its RCA outputs into my power amps. (Without a phono input or multiple audio sources, who needs an expensive stereo preamp?) Do these RPi endpoints have hardware controlled volume, and/or is there some way in Roon to limit volume coming out of a device? My power amps are 45…