When you say RFI, do you really mean mains hum? RFI is higher in frequency (because, you know, it's radio frequency). As the other guys pointed out, I think you're creating a ground loop with the second speaker connection.
Let's assume you have a fairly new computer. You will need a DAW software package. A lot of folks around here recommend Cuckos Reaper; it is free to try and only $60 to buy.
That and the gear you listed is pretty much the minimum entry point for recording and monitoring what you've recorded...
I built the Passive Reamp box. Incredibly simple to build. I may try the Colour modules in the future. Since I'm on their mailing list, I received some hype-mail about a 500 series Pultec EQ emulation that may (or may not) be coming as a kit. That could be interesting to me...
Are you sure you have the 2i2 selected (and correct channel 1 or 2) as the input to the GarageBand track?
And you do need the 48V phantom power for the Rode.
There are a number of DAWs that will run on OSX. I use Logic Pro X; I like it but I've never used anything else. There are a lot of Reaper users around here and it is available for OSX.
Get as much RAM as you can afford. My iMac has 8GB and is usable but I wish I had 16 GB. If you are...
I had on my to-do list to replace a curtain on a window that is maybe 5 feet behind and left of my mixing position with something that would be at least a little more absorbent. I took room response measurements with the existing curtain open, closed, the blinds open and closed, and with a bass...
I've found it difficult to use the built-in reverb on Kontakt instruments when I want other instruments to sound like they're in the same space. But EQ, compression, sure - whatever sounds good. I like a smidge of delay on the Session Horns Pro instrument.
Either computer will do.
You want a USB Audio Interface. You don't need any more than two inputs and could probably get away with just one input.
You need software on the computer to record your audio. There are a number of choices. If you use the iMac, you already have GarageBand and can...
If you think your recorded tracks sound good (you got it right at the source) but aren't all that interested in learning the art and science of mixing, you can always pay someone else to mix for you.
You can use the H2n as your audio interface (or "soundcard"). Plug the USB cable directly to your computer's USB port or a hub. It's been a while since I've done it, check the manual.
If you want to use a mic with a separate audio I/F, you will need a different mic with an XLR connector, and a...
Yes:
"CSR’s four reverb modules are now part of the T-RackS Custom Shop, which means they can be used in the T-RackS CS processing chain like all the other modules. They can also be run as single plug-ins in VST, RTAS, AAX and Audio Units compatible hosts, that support 64 bit operation."
64...