What are the maximum AC power consumption specifications for the REF1000, S300, and S300i?

Idle consumption is 15W for the REF1000, 10W for the S300 and 5W for the M300. Peak power consumption could be over 1000W for the REF1000, 600 for the S300 and 300 for the M300. Likely you will not exceed 50W of average power consumption even when listening very loud for a long period of time (watch your ears...).

Can I obtain the all range (300W) of the S300 if i drive it with an iPod?

If the iPod can put out 1.5Vrms then it can drive the S300 to maximum level. This would be very loud indeed! Given that the iPod works from a low voltage battery (<5V) It probably has a maximum output level of about 0.75Vrms. This would limit power output of an S300 to around 35 watts rms. Still a lot of power. If you need more level then running the iPod into a PRe3 or the preamp section of an S300i would certainly give you all the gain you may need.

Does the connection to a mac need any more than a USB? Is the sound improved from itunes USB as it does to CD/DVD?

Yes, you just need to connect the USB to the Mac and select the Audio CODEC as the default playback for the Mac. Sonic improvements are similar. Especially if you have stored the audio on the Mac as Lossless Compression. If you are playing highly compressed AAC or MP3 files then the file quality will be the limit, although still much improved over playback from the internal DACs...

How do I use Headphones with a bel canto integrated amp?

The best way to do this would be to get a simple headphone driver from www.headphone.com/products/headphone-amps/ for example and use the Record out from the preamp section into the driver. You can then mute the main output and listen only to the phones while adjusting level on the headphone amp.