I know you mentioned doing passive speakers with a separate amp, but I would really look at active or powered options as well. For example the Adam Audio D3V is a really small powered system with a built-in USB DAC. For $300 those little things sound amazing for nearfield listening.
You could also go bigger and get their T7V which is around $250 each so $500/pair. Larger in both size and sound, they also sound really great for the price, and still come in below your 20lb weight limit.
I was worried about active monitors sounding to sterile but after hearing these Adams I changed my mind. They are very enjoyable.