Help with bi-wirable efficient floorstanding 2.5K

Mirage OMD-15. 91dB efficient and bi-wirable. They also have excellent bass and you may not need the sub for much of your music. I have a pair, and although I'm powering them with an 85 wpc integrated, the damping factor is just 25 and yet bass is very clear, extended, and articulate. My speakers are in a high-ceiling open architecture living area, and the Mirage's hemispherical dispersion pattern fills the listening area uniformly and effortlessly.

You might also look at other efficient floorstanders from Canada that were developed with research from Canada's National Research Foundation, such as the PSB Imagine T, Paradigm Reference Studio 100, or Energy RC-70 Tower. That model Energy lists at $2200/pair and has a claimed in-room sensitivity of 95 dB (92dB anechoic). They also make the Veritas v2.3i at $3K/pair. It's awesome, but not as sensitive as their RC-70.

That's right. With SET amps, frequency extension drops like a stone when the impedance gets much below 6-8 ohms.

What you REALLY need if you keep your current amplification is a speaker made to work well with SETs such as the Zu Druid. They claim 101dB sensitivity (and even if they exaggerate, it's still at least 96 dB which is what you need) and a 12 ohm nominal impedance. The review linked below shows the impedance never dips below 8 ohms.

The list price is beyond your stated budget but you may be able to pick up some used ones. Be on the lookout for something like this. Here's a fairly recent review.

If the Zu Druid is anywhere near its claimed 101 dB sensitivity, then you shouldn't need to biamp.