Suggestions on Pre Amp/Amp or Integrated Amp

Your ’slimline’ marantz (your wife may prefer it’s size or looks) has front channel pre-out jacks, that allow you to add a 2 channel amp for the front main speakers.

the Marantz preamp section controls the initial balance of all speakers to each other, and the volume changes of all speakers, the ’better’ or ’stronger’ amp powers the front speakers.

The amp or integrated amp needs to have no volume control, or ’pre-in’ or HT Bypass, an input that lets the AVR control the front speaker's volume along with the surrounds.

AI quickie below,

I would ignore the specific models mentioned, just get the idea, then ask more questions. I would also let the Marantz power the center channel with the surrounds, and hopefully the sub is self-powered. Keep in mind, often advice is for bigger/more involved systems than 5.1. Mine is a small 5.1 home theater.

"Integrating a separate 2-channel amplifier into a home theater system is a common strategy to increase headroom and power for the front speakers, particularly when an AV receiver’s power output drops significantly when driving multiple channels simultaneously.  This setup allows users to offload the front three channels (Left, Center, Right) to a dedicated amplifier, ensuring consistent power delivery for Dolby Atmos configurations while freeing up the receiver’s internal channels for surround and height speakers.

Key benefits and implementation methods include:

• Enhanced Power Delivery: External amps often provide 200 watts per channel or more, surpassing the power available when a receiver drives 7+ channels, which is crucial for driving demanding speakers at high volumes without distortion. 

• System Integration: Users can connect a 2-channel amp to an AV receiver by utilizing the "Pre-Out" or "Main Out" connections on the receiver, effectively using the receiver as a preamplifier while the external amp handles the power amplification. 

• Audio Quality: Dedicated amplifiers often feature Class AB or Class D designs with superior signal-to-noise ratios (e.g., >120dB) and lower total harmonic distortion compared to integrated receiver amps, resulting in clearer dynamics and better control over bass frequencies.

Popular options range from budget-friendly Class D models like the Fosi Audio V3 or Aiyima A07 (priced under $100) to high-performance audiophile-grade amplifiers like the Monolith by Monoprice 2X (200W x 2, Class AB) or the Summit Hi-Fi A2 (600W x 2).  When selecting an amp, ensure it supports balanced XLR inputs for better noise rejection and check for 12V trigger compatibility to allow the external amp to power on/off automatically with the AV receiver."

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Even if you get a different AVR, make sure it has FRONT Pre-Outs, to do the above. Once you have a separate ’better’ amp for your front speakers, you can use it without the AVR for 2 channel music sources, CD, Streaming ....

I’d look at Emotiva. 

Your budget could get separate processor and amp at 250 WPC five channels driven vs. 50 W shared in the Marantz.

Even their HT receivers are much more stout that the Marantz.

If the room acoustics are only ok, don't waste your money. Get the functions you need, including power, and give the rest away.

5K for mostly movies? Hasn't been invented yet. I would ask a home theater forum. This is a hifi board.

Yeah if it’s mostly for more power/better bass you could just get a more powerful stereo amp and hook it to the front L/R pre outs on the Marantz as the NR1510 is underpowered for your CM8s.  You have a decent sub, and if you haven’t already done so you might look into optimizing its settings/positioning to make sure you’re getting the most out of it.  If you have a large room you might also consider adding a second sub, which has many benefits.  Best of luck.