Should I buy a power amp.

Sure, be glad to.  I think if you click on my name you can DM me and we can go from there.

I’m not sure which way you should go but I have some advice on the center channel. 

If you sit in 1 place in the middle the center doesn’t add much.  I know this from a lot of experimenting and professional experience.  It’s real value is when you sit off-center. 

Further, if you get a center a 3-way is usually much better than a 2-way, with horn loaded centers a possible exception due to low cross point. 

My experience is you are better off with rear surrounds than a center in terms of movie experience. 

Also, one annoyance of the modern Anthem line is they follow the Dolby specs too tightly.  Netflix for instance has a lot of Dolby Surround encoded content, which should have Surround and center, encoded as Dolby Digital 2.0.  Anthem MRX will ONLY play this back as 2 channel, turning off your center and surround speakers under any and all settings. 

Other brands, and prior Anthem processors, allowed you to apply Dolby Surround, or other matrix processing to 2.0 sources.  Not Anthem. It will do it for PCM and Amazon for whatever reason doesn’t do this.  

Note that the processor will say "Dolby Surround" even though it’s clearly NOT being applied.  It’s a significant bug that it displays this when its’ not on. 

This is how I know what the center channel does/does not do.  I was being double blind tested by Netflix!! :D  Sitting in my favorite couch position, I could not tell the center was not the source of dialogue at all.  At all, and I consider myself a pretty good listener.  Of course, it's easy to tell surrounds are not working but unless you know the source movie you don't know if they SHOULD be on.  It was listening while lying down elsewhere that I discovered this issue. 

  Hi my totally random two cents if I may. I was in your position with a 5.1 system. I had Klipsch Heresy II’s Vs your 8000II’s . I had a Klipsch RC center and RC sub. I used a Yamaha RVX receiver. I also used different non Klipsch speakers in my system and different amps but won’t get long winded. The point mentioned above about a different amp Vs more power is very valid as Klipsch ( like you have ) are bright. I found myself going more and more to a 2.1 setting because my Klipsch center was TOO BRIGHT. Back in that era of my listening a friend that formerly worked at a big box store said he pushed Polk products for that very reason. So I’d suggest that an amp with a smoother presentation would sound better and if you do go to a center speaker audition a few and find one that has a smoother more lifelike  sound for voices. Also remember how critical center channel placement is. Some users will have a pedestal that the center is on while watching movies. Fun fact “ the Heresy was created as a center speaker for the larger speakers in the Heritage line “. Happy Hunting , Mike B. 

I suggest you buy or build a riser for your TV so you can place a center speaker directly below the middle of the monitor. We used temporary stacks of books to make sure the viewing angle/light reflections/clearance all worked together.

I had matching wood, but it could have been painted or stained ....

A back and ends to support a tempered glass shelf for the TV, so a 6-1/2" tall center channel speaker fits below

Here’s a Klipsch Center Channel (that's metal artwork on the wall above the TV)

I actually prefer the smaller Jamo shown below, matching cherry wood and it’s sensitivity matches my DBX Soundfield 100 Fronts

My Sony AVR sounds darn good, people dogmatically support the idea of separates, HT Pass Thru for Center, .... I listen to my music system, and my video system, I’m never tempted to change. I often change my Sony AVR settings to force 2 channel stereo of streamed programs, and often prefer that to whatever Pseudo Surround someone has created.

Blu-Rays, with excellent 5.1 Sound are Awesome.

I was under the impression that multichannel HT surround sound amplifiers like Anthem MRX “require” the front left, right, AND center channel speakers.