The Klipsch S-2 ($50 list, $25 - $40 street) are in-the-ear phones with good sound isolation and very good audio quality for the price. Like most of the less expensive phones, they are somewhat bass-heavy but not too bad in this regard and the actual bass extention is impressive. Midrange and treble are uneven but better than you usually get in this price range. They are sensitive enough to use with portable equipment. Wiring is kind of flimsy. If used in an active fashion, they are not likely to go much beyond the one-year waranty.
You really have to go to the Shure SE-215 ($150 list, $90 - $99 street) to get professional stage monitors with very flat frequency response, superb sound isolation, secure and comfortable fit, low listening fatigue and user-replaceable wires. These less expensive models do not reproduce extreme bass or extreme treble but they are very smooth within their range, say from 50 Hz to 12 KHz. They are sensitive enough to use with portable equipment.