Planars/ Electrostats benefits over box speakers?

If it's not large it won't play loudly or have bass.

Rather than trying to describe how they sound I strongly urge you to go listen to a demo.

Sunnyjim, I have been a long time ( 20+ yr) owner and advocate of Magnepans. I picked up a used pair of Coincident Triumph Extreme II monitors and they outperformed my MG 3.7Rs in nearly every respect. Coherence, accuracy of timbre, and resolution were all clearly superior. The coincidents matched the Maggies with respect to tight, fast, articulate mid bass. I found the low frequency roll off started higher, but was a more gentle slope with the coincidents, so that there was actually more useful information below 40 Hz with the coincidents. The only aspect where the magpies were superior was in the range covered by the maggie ribbons.
The Coincidents with stands are substantially less expensive than the 3.7Rs. I highly recommend that you consider the Coincident TE IIs, especially if you are interested in a small footprint.

It's all about recreating the reverberant field. To do it effectively the speaker needs to produce full range sound (woofers in addition to tweeters) through the reverberant section and at a level approaching that of the front primary driver level.

A big benefit of panels for me is one that isn't talked about enough: Full size images! A grand piano is HUGE, and a good recording of one sounds that way through panels. Through most boxes they sound miniaturized. Panels also create a soundstage you are looking up at, rather than down on. The best deal in panels is the Eminent Technology LFT-8b. They aren't that big, 1' wide by 5' tall, and match better with tube amps than do Maggies, being an 8 ohm load rather than 4.