Hi David

I replied yesterday but it hasn't shown up - apologies if this winds up being a repeat.

If I understand your system correctly you will get the data off the PI via USB rather than analog out or streaming.

In that case, you need three elements:
1. DAC
2. Amplifier
3. Speakers

All in one
The KEF x300a is a powered loudspeaker with a built-in USB DAC. You'd plug the PI into the KEF and that's all you need. There might be other products like this

DAC/Integrated + speakers
Check out NAD, Cambridge Audio, Peachtree, Music Fidelity V-90 and the Wadia 151. Simaudio makes a nice DAC/integrated something very nice but at a higher price point. PSB makes great budget speakers and Stereophile recently gave a Polk and a Pioneer good ratings for money.

DAC + powered speakers
Audio Engine makes a DAC and a well-regarded pair of powered speakers that would pair nicely.

All separates
Skies the limit here. But you can start with the brands I mentioned above and work up based on our price point.