a tube friendly speaker is one with stable impedance such as 8 ohms nominal and efficiency of 90 or greater. Tube amps do not like speaker loads that dip into the 2 and 4 ohm range since they don't have the high current output of sand amp designs...this is all general information but it's fairly easy to tell from mfg specs if tubes can power the speakers correctly but of course there are exceptions. BTW I'm a SET guy with super high efficiency speakers....and if you go tube then the tubes themselves have different sonic signatures...tubes are great if you like to tinker with your gear and appreciate tonal body and accuracy over hyper detail.
