Getting 2 subs

my experience with large subs is that the frequency is to low and for me to hear close up , so is it ok to get 2 different size subs to cover a wider spectrum of bass or get 2 matching subs?
 

You’ve described  my successful approach to increasing the low frequency SPL in a sound treated 24 x 18 x 8 ft listening area. The front wall has 2 corner placed Rythmik subs supported by a pair of smaller REL’s as one on each sub wall on each side wall. My speakers are Voxativ single drivers — fast and midrange rich, but relatively bass deficient.

Just added a pair of Rel 212SX to pair with my Borresen's and they sound amazing. Room Size is 18 X 24 and they are more than enough! 

As long as both subs are sealed, you shouldn’t have any issues integrating them. Don’t mix a sealed and ported sub in your configuration.

If you don’t want your neighbor to get thumped and just want overall clean immersion,

a) For starters, use the Harman room mode calc and position yourself out of big nulls.

(As a "general solution", i.e., it’s hard to say much without measurements or knowing anything about your room dimensions. Sbir is an issue with subs as well..that dudes don’t think about often enough.)

b) Try setting your subs a bit nearfield i.e. subs behind your couch up against each side wall as long as your room’s not too wide. If the subs get too close to you, it will shake your seat/couch like tactile sub bass drivers and become distracting. If the sub placement line connecting your 2 side walls is about 2 to 3 ft behind your couch, it could put you in a immersive womb of bass without thumping your neighbor’s skull.

(There are many goofy thought processes on AV forums about pressurizing entire rooms with a platoon of subs, which will only pressurize the neighbor and your wife’s innards and gizzards. Wife could run off with the neighbor and start collecting alimony payments from ya...not good)

I’d think if another SB4000 isn’t in the budget an SB3000 shouldn’t be too hard to seamlessly incorporate into the mix between placement and adjustments so I wouldn’t sweat it. 

I have a pair of Matrix 802 happily assisted by a pair of SVS SB3000 with the preamp set for a 40Hz crossover. Along with an old Bryston 3B-NRB, I have had occasion to replay certain tracks to see if I’ve just been through an earthquake. The earthquake via SB3k was provided by the last 30 seconds of "Another Tune Like Not Fade Away" by Solex. The last two minutes of "Ontario Plates" by Do Make Say Think is another track that I thought was an earthquake.

I've experienced actual earthquakes, too many to remember, & the SB3k pair has fooled me a few times.

The SVS share a dedicated 20amp circuit and the Bryston has its own 20amp circuit (makes a huge difference). The SVS footies are a must unless you have a concrete floor.