being rear ported means obviously means that the port fires in the rear of the speaker. will this affect sound in your enclosed space? very much so. will this affect sound if you had them open in the rear to another room? very much so again. if i were you and you had the space i would upgrade to the rb-61 and you can flush mount them with the rear closed and they will sound just fine. plus i have read that they are quite an upgrade over the 51's for a minimal price.
Home Theater :
Onkyo TX-SR805, Emotiva XPA-2/5, RF-63 LCR fronts, RS-62 (black) Sides, RB-61 (black) Rears
DUAL SVS PB13 Ultras (real american cherry)
Pioneer Kuro 50"
Denon AVR-1713, WF-35 ESPRESSO, WC-24 ESPRESSO, WS-24 ESPRESSO, Velodyne Optimum 8
2 Channel Setup:
Klipsch 1985 Cornwalls in Walnut