I think that you may be wasting your time trying to improve on the HT experience by using separate amps. That Onkyo is an excellent HT AVR. You will just be complicating things and wasting money. You would be better off just buying a higher end AVR than to start messing with multi channel amps and trying to mess with the different sound and volume levels of different amps and all the different cables and space that the gear takes up. In addition, you will still be using the Onkyo's preamp section which will be the weak link of the system if you decide to go with higher end amps. I would rather see you buy an amp switcher for the front speakers and switch between your Onkyo for HT and the Nak gear for 2ch. This way, you will be able to use what ever preamp and amp you want for 2 ch Music (I use to use this type of system). Marantz and NAD make some great sounding AVR's and they would be good for 2ch music as well. Marantz also puts quality sounding phono preamps in their gear.

MARANTZ SR7007 7.2 125wpc 4K Airplay A/V Receiver | Accessories4less

MARANTZ SR6010 7.2 Atmos Full 4K AV Receiver Wi-Fi/BT/AirPlay | Accessories4less


TV music is going to sound flat. Are you running TV music to the receiver via digital inputs so that you are using the receivers internal dac? That would be the best route.

A three channel amps will have a left and right as well as a center or 3rd set of inputs as well as 3 sets of outputs. So you would run your front and center outputs from the Onkyo to the 3 channel amp for the front 3 speakers and use the speaker outs on the onkyo for the rear speakers.

What is your budget for gear and do you plan on doing computer based music. For DAC's IFI and Schitt Audio make some nice DAC's in the $400-500 range. But there are cheaper good sounding DAC's. Going the used route can get you a nice DAC.

Again with out knowing your budget, there are some very good sounding TT's in the $250-500 range such as the Pro-Ject Carbon Debut DC, Denon DP-300F, Teac TN-300.