- tube or solid state.....personal prefence here...tubes generally require more maintance.. alot has to do with matching it to your speakers...space, and power ( good point - better to have to much than too little)
-relability.. buy from a reliable mfg... chances are you might need service (higher for tube gear)and a mfg that has a good reputation for service and relabilty is REALLY important... if tubes, i would suggest to go with a really long term mfg with a great reputation ( BAT, ARC, Quicksilver) and not a start up company..
single end / dual differential inputs - important to optimize your connections and topology (for true balanced) and iec connection for power cords
impedence- should be considered to match with the pre-amp.
current capability - 100 wpc from a krell/classe/pass is going to have a differnt presenation than 100 wpc from a rotel/parasound/adcom. with that being said 100 wpc from most tube amps will even have more current than a good solid state amp (except the bass -"generally speaking")
mono block vs stereo - depending on load, design, and budget...mono blocks "generally" sound better and look really cool
there are more points - hope this helps out
