Pete, IMO it's entirely possible for the PM800, in conjunction with a decent sub (you don't specify yours) to deliver smooth, full-range, accurate sound in a modest-sized room at reasonable seating distances. In a large room, they might not be as capable.
How big a room are you trying to serve with a pair of PM800s, and what's it like acoustically?
Room correction can do wonders, but it works better if the user has a good understanding of what it's doing and when it might be making mistakes. There are all sorts of factors that Audyssey or MCACC or YPAO can correct for -- 1/3 octave parametric EQ, standing waves, room reverberation, etc. -- that are difficult or impossible to modify without it. IMO it's the single most important advancement in home audio technology in the last 20 years. It doesn't replace knowledge and understanding of audio basics, but it can actually do things that no user can accomplish without it, that used to take thousands of dollars worth of specialized equipment and expertise to implement.
But if you don't have it, you have to do something else, and in a 2.1 system, where the problem is bad blending between the sub and the sats, that's almost certainly going to be moving the sub. If, as you say, you can't do that, then I don't know. You said that changing the xover to 80Hz didn't change the sound, which tells me that upgrading to PM1000s might not change the sound either.
The passive radiators in the PM series really do give them the sonic characteristics of conventional bookshelfs with larger drivers. Given the same crossover frequency, and the same sub in the same location, I wouldn't assume you won't have the same issue with a larger speaker.