I haven't heard the LS26 but I own the 36.5 (single box) and had the Ref 3 in my system for a few days. Both are great linestages with different strengths. I'm not sure which unit I would prefer over the long haul although the Ref 3 with about 250 hours on it may have needed more time.
The Modwright excels in low level resolution/ambient detail, imaging, and refinement. It's sound can also be altered by changing the rectifier. The GZ34/5AR4 is standard but it can also take the GZ32/5V4 and
GZ37/5U4 families. Upgrading to NOS is highly recommended.
The ARC throws a bigger, more enveloping soundstage, has more bass impact and stronger macro dynamics.
This is obviously an unfair comparison as the single box 36.5 retails for $5k and the Ref 3 for $9k. However, from all reports the Dual Mono 36.5 ($9k) would have all of the advantages of both the above pieces. There's a used one being offered for sale now.