The SFB 1000.4D is not a subwoofer amp. It's for the midbass and tweeters. it's one of the most powerful 4 channel amps on the market.
350 x 4 at 4 ohms,
590 x 4 at 2 ohms,
1050 x 4 at 1.
It's really designed for people that are going to run multiple PA/PRO style mids/tweets or low impedance 6/8" midrange drivers. It's a beast for sure.
As for your subs, I think that the 3500 watt you have is more than enough.
Considering you have to go to 28000 (yes that is twenty eight thousand) watts to double the volume from that 3500 watt amp, kind of silly!
3500 to 7000, 3db or 33% louder volume
7000 to 14000, another 3db or about 66% louder
14000 to 28000, another 3db or around a doubling of the volume as perceived by the ear, again, not really practical.
Going to a 5K amp would net you about a 16% increase in volume, and really about as far as you can take it before having other major considerations.
Things start to get complicated when you start introducing that kind of power specifically for driving subs. Better electrical, lithium banks, HO alternators, multiple cable runs, parallel grounding, lots to consider.
3500 watts to a couple subs that can handle it is pretty darn loud.