I’ve had 2 Sonos Connects for about 3 years and am pretty happy with the way they work — easy setup, reliable connections, sounds pretty much like CD quality to me. However, what is the next step up, without having to sell the kids for medical experiments? Gear list: Couple year old iMac with SSD and 500+ CDs ripped as lossless flacs to 3TB Apple Time Capsule, which links via WiFi to the two Connects. One Connect goes RCA out to the Aux jacks on an old-ish NAD C320BEE w/ KEF Q65s. With concrete floors, the acoustics aren’t great, so my more serious listening is to the other Connect with coaxial out to a Schiit Modi Multibit DAC and then to a Rega Ear headphone amp with Audioquest Nighthawks on my head. Again, where next with the most bang for my buck? Also, I don’t have any hi-res files. Yet. I bypass the Sonos interface and Roon it at all times.
Take a look at some of the RPI3 players. I have a HiFiBerry DAC+
They come in DAC and Digital coax models. There are other brands, but that’s the one I have used. Under $150.
Roon RAAT capable.
Unless you already have a full backup of your music I would spend on that part first.
RPi dacs are many and most these days will eat an old Sonos for sound quality IMHO. As a side benefit they will be using RAAT too. Dont forget to use a good power supply too.