Everything being equal weight can be a good guideline but everything is not always equal especially when comparing models from different manufacturers. In this case, the 805seems to offer more power, though people could still say they are comparable by arguing that doubling the power only yields 3 dB SPL.
I found some lab test results for the 875, the 805 weighs the same, both a little lighter than the 745 and all 3 are much lighter than the 7300 but the Onkyo 875 is slightly more powerful than the HK 7300 in Stereo while slightly less powerful in 7 channel.
Onkyo SR875
http://www.soundandvisionmag.com/receivers/2463/test-bench-onkyo-tx-sr875-av-receiver.html
Output at clipping (1 kHz into 8/4 ohms)
1 channel driven: 201/322 watts (23/25.1 dBW)
5 channels driven (8 ohms): 141 watts (21.5 dBW)
7 channels driven (8 ohms): 128 watts (21.1 dBW)
Distortion at 1 watt (THD+N, 1 kHz)
8/4 ohms: 0.02/0.03%
Noise level (A-weighted): -76.1 dB
Excess noise (with sine tone)
16-bit (EN16): 0.7 dB
Frequency response: 20 Hz to 20 kHz +0, -0.8 dB
MULTICHANNEL PERFORMANCE, ANALOG INPUT
Reference input and output level is 200 mV; volume setting for reference output level was -14.0.
Distortion (THD+N, 1 kHz, 8 ohms): 0.01%
Noise level (A-weighted.): -82.9 dB
Frequency response: <10 Hz to 183 kHz +0, -3 dB
STEREO PERFORMANCE, DIGITAL INPUT
Reference level is -20 dBFS; all level trims at zero. Volume setting for reference level was 6.5.
Output at clipping (1 kHz, both channels driven)
8/4 ohms: 185/320 watts (22.7/25.1 dBW)
Distortion at reference level: 0.02%
Linearity error (at -90 dBFS): 0.2 dB
Noise level (A-weighted): -75.6 dB
with 96-kHz/24-bit signals: -88.8 dB
Compared to the much heavier HKAVR7300,
http://www.soundandvisionmag.com/assets/download/harmonkardonAVR7300lab.pdf
Output at clipping (1 kHz into 8/4 ohms)
1 channel driven .... 202/343 W (23.1/25.3 dBW)
5 channels driven (8 ohms) .....145 W (21.6 dBW)
7 channels driven (8 ohms) .....139 W (21.4 dBW)
Distortion at 1 watt (THD+N, 1 kHz)
8/4 ohms .........................................0.03/0.04%
Noise level (A-wtd) ............................ –75.0 dB
Excess noise (with sine tone)
16-bit (EN16) .........................................+1.4 dB
Frequency response
20 Hz to 20 kHz +0, –0.3 dB
STEREO PERFORMANCE,
DIGITAL INPUTS
Reference level, –20 dBFS; all level trims at
zero. Volume setting for reference level, –10.
Output at clipping (1 kHz, 8/4 ohms, both
channels driven) .....181/290 W (25.6/24.6 dBW)
Distortion at reference level .............. 0.02%
I can't find any data for the 745 but I doubt it will be much more powerful than the 630, which is available. online. Between the two, I would take the Onkyo 805. I thought it offers more bangs for the buck overall. I would expect the 805 to win by a wider margin in 2 channel output so if your main concern is 2 channel music, the choice is even more obvious. I also do not believe this thing about HK being warmer, more musical is true/objective. It is important that you bring your own favourite CDs and take the time to listen for yourself.