Very nice musical speakers you have. They are also reasonably efficient at 90 db and 8 Ohms. If they were 4 Ohm, normally I would say the addition of an external power amp would make a huge difference. Since we don't know what Onkyo AVR you have, this may not be the way to go. Onkyo makes some real beasts and you may already have all the power you need.
I have been down the road your on, adding power amps to AVR's to up my 2 channel performance and what I found made a bigger difference, was a 2 channel pre-amp with HT bypass. I find the 2 channel "pure direct" modes mostly an afterthought in AVR's. If you think about it, why shouldn't it be. AVR's are designed for HT applications and multi-channel; not 2 channel stereo! For superior stereo performance, 2 channel components still rule. Just look at Audiogon.
Now I know you can't afford separates at the moment but here is another option. Get a quality 2 channel integrated amp with HT bypass and now you'll have a dedicated component, designed to properly reproduce 2 channel, that can also function with your AVR for your HT listening when called upon.
I recently replaced a Parasound 2100 pre/5125 power amp embedded in my HT system with a single Marantz PM15S2 LE integrated and the this unit has brought my 2 channel listening to new heights. I am driving 4 Ohm Vienna Bach Grands with it. The AVR is used for the center and surrounds for the occasional movie that I watch. I offer the above as another suggestion as it certainly worked for me. Music has never sounded better in my system.