One modest cost alternative is to add HD's Tour Pak Bass Port kit with the dealer download to reprogram the amp to provide more bass.
When you are considering amps, none will work the same as your OEM amp. Specifically, the fader is controlled from the amp, not the HK head unit. None of the aftermarket amps work this way; the fader is provided from the head unit. With the wiring on the newer CVO Ultras, switching to a 4-channel amp requires some relatively simple rewiring inside the fairing to access the rear speaker pin-outs of the HK head unit that aren't used on the '10 or so newer CVO Ultras--they simply use the front speaker outputs; route that to the amp; and, the amp splits the audio signal for front and rear speakers. To use an aftermarket 4-channel amp, you'll want to access the rear speaker pin-outs so the fader will work.
The OEM amp provides 40W per channel, 4-channel. By comparison, the J&M 500W amp provides 125W per channel using 2 ohm (Rokker) speakers.
I suggest you talk to guys who have done audio projects that may interest you, or have a lot of experience with audio mods--starting a thread is fine; but, typing a comprehensive reply takes a lot of time--a phone call is a much easier way to convey information and better understand what you're after; your budget; and, your willingness to modify what's on your bike.
Good luck with your search.
Carl