Sub Question. One higher quality sub or two lesser quality? (Rythmik vs HSU)

I used to have a HSU 3 Mark lV subwoofer and I know there are better but I would buy no other . It's well made and designed , goes very low with very little distortion , has very high wattage amplifier and has all the connections necessary for any need . HSU makes great subs . Rythmik also . Rel is also recommendable .

The issue is never the quality of the sub.  The issue is always how you integrate it.

I would also take a look at a pair of SVS SB3000 subs. Right at your budget point and an excellent value. Or get 4 SB-1000s and make a Swarm!

I agree 100%

Learn your room and speakers and measure your response below 500 Hz before you do anything. Most subs just output distorsion below 25 Hz when playing loud. Professional digital correction of your Cornwalls may give you quite another listening experience.

It’s been said a million times there is no substitute for quality

And always worth repeating. I would rather have 1 higher quality sub than multiple lower quality subs. Add another when finances allow if you think you need to.