@sharza Excellent work so far, but a few things you need to know in order to consolidate some of the maths and apply it to real world use.
1. The calculator I linked to is assuming one speaker in open space, as you mention above, multiple speakers raises SPL, and so does in room response, as the walls and ceiling will reflect the sound and make it louder. There is another calculator here that attempts to take account of some of those effects, it is a lot of estimation though:
2. Speakers being run at 300 watts to sound their best, is a bit of a grey area, it depends on the dynamic range of the source material, the distance from the speaker, and the upper SPL level you want to achieve. Having said that more headroom is always better, so that in those dynamic peaks, there is enough power to play back the audio nice and cleanly with no distortion.
3. Your AVR is specced at 105 watts into 8 ohm speaker when running 2 channels. When running 4 channels (bi-amped) the power output per channel will drop, to approx 80 watts per channel. It is the bass frequencies that take the most power, so potentially by bi-amping you are reducing the power going to the bass drivers of the speakers.
When I wondered if I needed a power amp, I bought a very low cost power amp to try it out. This is an option for you, and I recommend a Behringer A800, or maybe two which you can run in mono mode, one for each speaker:
Reference class power amplifier, RMS (4 ohms) per channel: 400-449 watts, RMS (8 ohms) per channel: 200-249 watts, weight: 3.2 kg
www.dv247.com
This is a review and detailed measurements of the Behringer A800 Class D Stereo Amplifier. It is on kind loan from a member. The A800 costs US $299 including free shipping from multiple sites. The member who loaned it me has done a youtube video of it which you can see by clicking on this...
www.audiosciencereview.com