How do you amp your system?

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.

Oops, Monday moment - it is a 9.2, this is on the factory website: 150W per Channel (8 ohms, 20 Hz-20 kHz, 0.06 % THD, 2 ch driven).

My 2 channel @ 125 does run out of gas. When I'm in 'play mode', I have the volume up to 0.0db and up, with a max available of 16.5 on the AVR. Normal TV is about -22(+/-). These numbers are on the AVR screen when I adjust volume. At these levels, the sound just gets bad...detail goes away, stage collapses, etc. The Kappa Series was a power hungry series of speakers, though, but even so, I definitely need more.

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.)

The Kappas will be upgraded. One day...but they aren't biampable.

I've read about Parasound, ATI, Outlaw, Wyred, Emotiva....the options seem endless. There's even a used Krell KAV500 on eBay that isn't too far out of budget, and it's bridge able to 400wpc then when I add a center and upgrade to the PSBs which are biampable, then I could use two channels each main speaker with 100wpc and still have 100 for the center...or there's the 200wpc+ with a few other brands....

That's where I start to get lost, because of the amount of choices with no 'right' answer

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.)