Sure, maybe, dunno.
1000w Amp tells me nothing. Is it a home stereo with 7 channels at 120w each or a car audio amp with 500w per channel. Is the Amp rated at Peak to Peak , or Peak, or RMS.
I'll give an example. Car amp. Rated 1000w Peak to Peak in Bridged (Mono) mode. Generally, that's 500w per channel. Being Peak to Peak (P2P) you only use the top half of the sine wave (Bass hits at the + not the - ) so really, that's a 250w Peak amp. Out of that 250w, anything beyond about 70% is distortion, so honestly you are trying to power 2100w subs with a 175w per channel amp. Good luck with that.
Or.
That amp is 1000w per channel RMS value. Take it at face value, that's 500w per channel undistorted power.
And.
2100w subs. Same applies. Subs don't hit on the -, just on the + , so that 2100w sub is realistically nothing more than a 1050w Peak sub, or 700w RMS.
You put 1000w Bridged mono P2P into a 700w RMS sub, that's 175w RMS into a 700w RMS sub. Not exactly very loud.
You put 1000w Bridged mono RMS into a 700w RMS sub, turn the dial to 7 (1-10) and 2 of 3 things is guaranteed to happen. You'll blow out the car windows or roast the sub coils. The guarantee is blown ear drums.
The simple answer is yes, you can use 2100w subs on a 1000w Amp, but depending on the values, you might get kinda quiet, or extremely loud response, no telling.