Integrated to Separates Advice

You are going to take a hit - how big a one and how much you notice depends on how loud you typically listen to the front channels and the size of your room. There is no way the Marantz is going to match the KW500 for lack of compression at volume, woofer control, and nuanced performance. In your favor, the Focal's are very efficient, but you will still notice a difference in dynamic head room and detail retrieval in any case. I would think to come close to matching the performance of the KW500 for two channel listening, you would need to go with the Denon A1HDCI pair of separates, but that is getting incredibly expensive - 15 grand retail.

I might suggest replacing the mid-grade Onkyo with the top Marantz, Onkyo, Sony or Yamaha receivers and ditching the KW500 for either: a large integrated that provides HT bypass (e.g. Plinius Hiato), a good but relatively reasonably priced two channel power amplifier (e.g Parasound JC1 or PLINIUS SB-301), or a pair of Musical Fidelity Supercharger 550K's in line with the receivers front outputs. I think the big integrated will be the best for you because it completely bypasses the multichannel preamp in the receiver for 90% of your listening (assuming you have a dedicated two-channel source hooked up to the integrated), and would allow you to use the superior amplifier in the integrated for the front speakers in HT mode. As good as these new AV receivers and even separates are, they cannot run with the better dedicated two channels integrateds IMHO.

One other important consideration. The preamp section will be as important as the amplifier section of your receiver. One of the reasons the KW500 sounds so good, even at low volume is the solid preamp section. The other is the capcity of your large amp to effortlessly and quickly respond to dynamic swings in the music. You are a little spoiled now because you are enjoying high quality components in both sections, and the superchargers in combination with an AV receiver will only address one of these.

If you go down the receiver/supercharger path, you should be careful to select a receiver that sounds best at the low levels you prefer, AND has the capacity to shut off all unecessary circuits like lights, DACs and video boards when operating in two channel for audio. Those things are very noisy and do more to compromise audio performance than most people appreciate. Pretty sure Denon has this feature on their AV receivers, not sure if all the others have it as well.