Can single ended 211 or 212 amplifiers drive magnepan 20.7?

Correct on all counts @thegreenman! Modjeski was able to coax 100 watts out of a single pair of KT-88 tubes not by driving them hard, but by doing just as you suggest---creating a wonderful circuit. You can read all about the amp (in both original and Mk.2 iterations) in the Stereophile reviews by Michael Fremer.

@assetmgrsc: To achieve more than "pretty good" sound with a sub augmenting your Maggies, look into the OB/Dipole Woofer offered by Rythmik Audio in conjunction with GR Research. Magnepan has been working on a dipole woofer to mate with their speakers for a few years now (Wendell Diller has always insisted that omnipole subs---dynamic woofers in an enclosure---"do not work" with dipole loudspeakers), but no need to wait.

As for a tube amp to use with Maggies, Roger Modjeski designed his Music Reference RM-200 tube amp specifically for low impedance loads. 100 + wpc from a pair of KT-88 tubes per channel, 5,000 to 10,000 hours lifespan. The amp resided in the Stereophile Recommended Components List for a decade, Michael Fremer’s "reasonably priced tube amp reference." Out-of-production (though that may be changing), but available used for from $2,000 (original iteration) to around $3,000 (Mk.2).