A cap (C.A.P. is a brand) allows the voltage delivered to the amp to remain higher more of the time.

It smooths out voltage fluctuations that are associated with bass notes and higher amp draws on the power supply.

When your amp uses a loosely-regulated power supply - like most do - it can deliver more power at 14 V DC than at 11 V DC (at the power supply input terminals). The cap keeps that voltage higher, thus allowing the amp to make more power more of the time.

When your amp uses a stiffly-regulated power supply, it usually gives you the exact same amount of power with a power supply voltage of 11V to 16 V. Old PPI, old xtant (and some current, but I don't know which ones), some Zapco, old Hifonics/Autotek, and current JL "slash" are all stiffly-regulated.

When you have a smaller battery, caps help more...when the battery's at the other end of a long wire, caps help more.

So basically, when installing JL "slash" amps in an X5, with a huge rear-mounted battery requiring a very short wire, you have all the ingredients for not benefiting from a cap at all.

I'm not saying that one is better than the other overall. Research dynamic headroom as it relates to power supplies sometime. But if you have the budget, big (not medium - big) stiffly-regulated amps are my preference.