The QA451 is the replacement product for the QA450. The QA451 is designed for high-speed testing of amplifiers. If you are testing at the module level (with modules designed for single supply operation), the QA451 allows the on/off control and current measurement of the supply rail. If you are testing finished amplifiers (eg amplifiers that plug into a wall), then the QA451's programmable loads can enable fast automated testing when combined with the QA401 audio analyzer.
The QA451B replaces the QA451. The products are the same, but the current sense resolution for the DC supply switch has less resolution due to silicon availability issues related to the Allegro part used on the QA451.
NOTE: Exceeding the power handling of the QA451 will result in internal load resistors opening up. These resistors are very difficult to remove without special equipment for uniformly heating the board. The manual goes into great detail on power handling capability of the QA451 and how to compute a thermal budget for factory test flow. Do not purchase the QA451 if are looking for a load that can soak up hundreds of watts while tuning an amp. The QA451 isn't designed for that. See the thermal discussion on page 14 of the manual. Alternately, you can use external loads and use the QA451 and a balanced to single-ended converter with a steep low-pass filter (Fc=65 kHz). See the link below about External Loads.