Digicidal hit the point.
ruler flat eq w/ THD 0.00001%, it's very much achievable. enjoyable? it's a completely separate question. I have a Ascend sierra-1 (recently Nrt upgraded), pretty much flat, and sound wonderful. Mate it w/ a highend SS amp, it's great.
BUT the true allure comes from my other small SET 300B setup w/ FR spks. measure horribly, and high cost to buy/maintain, but it's just magical w/ female vocal.
Frankly there are not that many FR manufacturers (meaning they actually make their own driver, not just using off the shelves) with prices like Zu. I really hope they success and turn the general audio-head to FR.