How do you amp your system?

Far from "prevailing wisdom," my 7 channel system evolved from my 2 channel stereo.

I started with 2 speakers driven by 2 monoblocks. Each speaker had its own subwoofer to fill out their low ends.

When I added a screen, I routed the 2 front channel outputs of the HT processor to be one of the inputs to the preamp of my stereo. And I added a center channel and 2 rear speakers driven by three other monoblocks, all sourced directly from my HT processor. These monoblocks are powerful and accurate but do not have the same class of sound as my main front channel monoblocks.

More recently, I added 2 more channels for a 7 channel HT. The 2 added speakers became the rears and the former rears became the surrounds on the sides. My rears are more efficient than the surrounds so I used a lower power stereo amp that I had bought some time ago. Fortunately, it is made by the same designer as my main front channel monoblocks, so at least it does share the same character of sound, although this is of minor importance given the sonic material usually sent to the rears.

Since my main fronts both have subwoofers and all my speakers are close to full range, I don't feel the lack of not having a dedicated subwoofer channel. When I set up my system in my HT processor, I input that there was no subwoofer for the ".1" channel.

In my experience, the 4 surrounds+rears won't require a whole lot of power, while the two front mains will need the most support. And for some recordings/movies the front center channel does a lot of the work. (I believe your ULS15 sub has a built in amplifier?)

Accordingly, I would try keeping your 2 channel amp for the 2 front main speakers -- they must have been working pretty well together so far?

Can you use the Yamaha AVR to power the center and the 4 surrounds+rears? I figure the center will require the most power and the other 4 won't be very demanding. (By the way, I don't quite understand your description of how the Yamaha supplies power -- a spec page I glanced at identified the AVR as putting out 150W x 9. Are you saying that only 150 W is available to all speakers at any one time?)

If you find the Yamaha runs out of gas hooked up to 5 speakers, I would recommend getting a monoblock for the center channel and leaving the Yamaha supporting the 4 surround+rears.

Well, if your 2 channel amp doesn't give you enough and you are OK with biamping the Kappas, then perhaps adding another 2 channel amp and using it along with the old 2 channel amp in a biamp set up should get more volume out of your mains.

The Yamaha AVR would still be driving the center and 4 surrounds+rears. (Again, adding a monoblock for the center remains an option if the Yamaha can't handle your needs.)

Sounds like a plan.

As you say, there's no right answer.

I can only add that you may find yourself on an upgrade spiral that could involve buying and selling a lot of equipment. In the past I have found myself to be better off just jumping over a couple of go-rounds. (Less fun but less work and less expensive.)