The HTR and RX series are basically the same thing. HTR is the code they use for some of the big box stores. For example, the RX-663 is the same as the HTR-6160.
For around $400, you have a few options. The Yamaha 663, Pioneer 1018, HK254, or Onkyo 606 are all good picks. You could also try for a Denon 789 using the 25% coupon code at 6th Avenue. If you're trying to power a passive sub, I'd probably go with HK, Denon, or Onkyo which have stronger amp sections.