sub is just a box speaker able to go low and crossover filter is , well i guess what is called sub bass and two of the same is best and can be cheap doesn't have to cost thousands few hundred cheap on ebay is good enough nothing wrong with second hand used , or diy
but mix subs with other subs is doable and one at front and one at back of room evens the bass out and cheap behringer DCX 2496 is must have then can add 6 subs with single LFE.1 input or use two inputs A left stereo sub and C right stereo sub , use B for LFE.1 per , channels outputs 1 2 3 4 5 6 and use nine peq filter bands with maybe cheap behringer FBQ 2496 daisy chained channel A to channel B so to maximum the peq bands to 40 should be enough to smooth the frequency response , with some trail and error as it would take while
behringer class d amplifiers nx3000d 6000d series which seem to be popular
it can be done cheaply