David Berning ZH-270 otl definitely stable into 4 ohms, no problem. I am currently using them to drive Soundlab Pristine ll's with outstanding performance, the 270 can drive then up into the mid 90db's without strain and they drop down to at least 3 ohms. This is the only OTL on the planet that is able to actually increase its output power steadily as impedance drops. 70W@ 8 ohms (typical output power at onset of clipping 84W@ 8 ohms, 110W @ 4 ohm). This amp has the MOST drive and musicality of any amplifer I have ever heard. It also incredible bass output, unlike other OTL's.
Chris
Chris