Those 2000uF NPE caps prevented the bass go lower, additional woofers will give you more bass but won't help much to get lower.
You can use a class d pro amp with active crossover to low pass at 80Hz or lower, directly power the additional woofers (bypass the passive crossover) to get deeper bass.
https://www.crutchfield.com/S-IQfbmg9ktUj/g_394150/Pro-Amplifiers.html
You can use a class d pro amp with active crossover to low pass at 80Hz or lower, directly power the additional woofers (bypass the passive crossover) to get deeper bass.
https://www.crutchfield.com/S-IQfbmg9ktUj/g_394150/Pro-Amplifiers.html