I think the HTR's can be purchased online with full warranties, while the RXV's need to be purchased at a local audio shop for a valid warranty. The 5990 and 1600 are almost identical twins, while the 2600 is a step above.
The 5990 is actually 120 watts x 7 (at the same thd/impedance) as the 1600 if you check the manual. The 2600 is 130 watts x 7 at those specs.
HTR's are targeted toward big box stores, and RXV's are targeted towards higher end shops.