Yamaha describes different series of receivers, prefixes RX-V, RX-Z, & HTR. Units in the HTR series appear to be identical to those in the HTR series, judging by features, appearance, weight, dimensions, and cost. Are identical units in fact offered in both series? If not, what are the differences? Also, are Yamaha receivers reasonable choices in the lower price ranges, for instance the HTR 5550?