At one point, with a budget similar to yours, I was planning to build a set of speakers based on Boehlender Graebner "ribbons", similar to Genesis and Wisdom designs. More like the Genesis with an array of ribbon tweeters since the Wisdom ribbon, which handles high frequencies better than the BG and earlier Carvers, is proprietary. However, a pair of Genesis 350's popped up for less than it would cost to build and probably far better than any feeble attempt of my own. If you weren't aware, Genesis is the company Arnie Nudell formed after selling Infinity but relatively few people have ever heard the bigger ones outside of CES and other shows.
The BG ribbons are available from Parts Express and the new DAB-1 active xover from Wisdom handles the necessary notch for resonance to extend them down past 100 Hz. From that point, an active sub or something more DIY would take over.