Oh dear, I'm not sure who recommended to you the Rockville SP128 12's, but I could have told you they were going to be bad for your application. It says right on their website they are intended as a PA system for DJ's. 800W! Such a large woofer and a tiny tweeter rated for that power output together would be useless for basically anything but the intended application. The huge woofer carries the low frequencies of the voice, and the tweeter the high frequencies (t, s, k, etc.) at very loud volumes. It says they have a crossover, but clearly the one they use is deficient for anything but the intended application. The squeak is probably the woofer struggling with high frequencies the crossover failed to filter out.
https://www.rockvilleaudio.com/spgn128/I suppose I can blame myself a bit for your mistake though, given the advice I gave you above, so sorry about that. Unfortunately it is quite a bit more complicated.
There are so many speakers out there, and each of us has their ideas of what would be good. If you must get a passive organ speaker, get a hifi style one with at least an 8' woofer, a smaller woofer (maybe 4') a tweeter, and it must have a decent crossover for all three drivers. You don't need it to be loud, I would get it 300W at the most. If you need a 12' woofer, then make sure it has a smaller woofer (4-6'), and a tweeter with a decent cross over. If any driver is fed frequencies that they don't reproduce well you'll get distortion, and the louder it's driven, the worse the distortion will be. In my opinion, the ideal is actually a relatively low wattage speaker, like the ones Allen uses. My Allen HC12's use at most 50-60 watts, but they have a huge 12' driver, two 4' drivers, and a tweeter, with an enormous crossover. Each one weighs like 200 lbs. It's almost impossible to raise the volume so that organ music sounds shrill on these, which is the point. The highs are clear but never shrill, and the lows never get overpowering, even though the woofer is 12'. The cabinet is super heavy and well padded inside, so you don't get cabinet vibrations. The secret is the extremely effective passive crossover which is perfectly tuned to the drivers, and drivers that cover a lot of different ranges well. The problem you're having with your Allen monitors is almost certainly foam rot. If you replace the surrounds, you'll see that they sound much better. Just be careful not to drive them too hard. Another possibility for old monitors with passive components is that the crossover components have started to drift. You may need to get your crossovers rebuilt (usually it's the resistors).
Another possibility is that you're using the wrong Allen monitors for the job. Sometimes they put out monitors that were designed just for an antiphonal division or something similar (basically just trumpets). You'll never get a proper sound for the full division out of one of those.
You don't need Allen HC12's, but you do need a set of passive speakers that are designed for relatively low wattage reproduction of a very wide signal band without distortion, and the best for that is to have at least 3 drivers, one for the middle, one for the bottom ish, and one for the highs. If you have to get one with two drivers, then I wouldn't get one with a woofer larger than 8-10' and for the top one a 4'. The bigger the spread, the more likely the crossover is going to struggle to filter things properly, and for organ music, you get a lot of signal in the band between an 8' woofer and a tweeter that you're going to struggle to reproduce well. A 4' driver usually goes close to the top pretty well. I've seen some Hauptwerk contributors use nothing but a 4' driver for the manual divisions! I wouldn't do that, but they say it sounds pretty good! No matter what though, the biggest woofer will squeal the high frequencies unless you have a proper crossover, so don't cheap out on that!
Finally, it's a good idea to look at the intended application. PA type monitors almost always perform poorly with Hauptwerk, so that is a start. The best are Hifi type. Home theatre units sometimes work ok (mostly for subs), but sometimes they are missing robustness in the middle frequencies where you want them for Hauptwerk too (they tend to emphasize sound effects, and for that they need more power at the higher and lower frequencies).
Within the context of all that, it's a good idea to get a decent sized driver if possible, 8-10 inches at least, because at the bottom end the lowest pipes of an organ tend to produce a fair bit of bass frequencies, but again there are a lot of other factors at play. Those bass pipes are not as often played though, so also keep that in mind!
I feel like this post is rambling quite a bit, and feels a bit self contradictory at times, so sorry about that! I do feel a bit guilty for potentially misleading you though! It wasn't my intention. Sometimes it's challenging to communicate a complex system like this, and a complex answer that teaches the underlying principles is sometimes a more useful one.