I disagree with all of the responses so far, except for Lonman. Yes, a good DSP is a good tool, but the OP must learn how to use it and have the correct expectations about what it will do.
The HPR151i has a basically okay line-level crossover built-in, and when connected correctly with the OP's HPR121i tops, the main benefit of a DriveRack PX would be the ability to apply and save corrective EQ for the loudspeakers.
The auto-RTA function sucks. Yes, there are people who like it. No, it really does not do good things a lot of the time. It's placebo effect. Here is a good article by Bennett Prescott explaining why these things are pretty bogus:
http://www.prosoundweb.com/article/prin ... _your_ears
The AFS function might be okay, but it also might guess poorly and eliminate useful sound, while not eliminating feedback. You should not rely on these things. Learn to do it with your ears, because a computer cannot tell the difference between feedback and legitimate program material. It guesses, sometimes well, sometimes badly.
The sub-harmonic synth is, well, a toy. Cool marketing gimmick. I know zero pros who use these things for any purpose in a live setting. Might be cool as an effect in the studio, though.
The posts so far have hit on all the "marketing gimmick" features of the DriveRack and none of the features that are actually useful, and why it is a good tool to have and good for the price:
* many presets can be saved for different PA setups, rooms, etc.
* input and output EQ for speaker and room correction
* enough delay for loudspeaker alignment (not enough for delay stacks)
* good, flexible crossover that is easy to use
* input and output gain adjustments
* peak limiter and compressor for protecting speakers (if you know how to set it up with your amps/speakers correctly)
* useful EQ presets for many common speakers
These are all good features and easily worth the money it costs. The gimmick stuff that every other poster has touched on is sometimes useful, but also, sometimes does more harm than good. IMO that is a bad tool, because an experienced user does not need any of those things, and a an inexperienced user probably can't tell the difference.