Well if you don't have the need to record all 4 tracks at once that opens up your possibilities. Almost to a point where it's impossible to choose.
That original Mackie you had will probably do just as good of a job as anything in the lowerend. There are other weird options like Black Lion Audio's
A/D converter with spidif and AES out.
But really if I had that Akai and only had a soundcard that could do stereo, it would drive me nuts personally. I would eventually want to inerface
the thing with the computer for something. And who knows maybe you will come across some quad reel gem at a yard sale and can transfer it for us
I am having a hard time giving solid recommendations as I haven't used a mac for years. I will say that the surround sound support on Vista is nothing
to write home about so I doubt either have an advantage one way or the other. I usually have to switch over to XP if I want to use any consumer
surround formats. Maybe it is just a codec issue and I haven't gotten it sorted out.
And well balanced with unbalanced. I will give you the easy answer first. Go down to radioshack, or if you want something quality order it online or go to mom and pop electronics store (think switchcraft, neutrik, canare) and get 4 RCA > 1/4" mono adapters. You can also do the inverse and not hear a difference which would be 1/4" (TS or TRS which ever you have the option of, if you can use both I recommend TS actually) to RCA adapter with a 1/4" TS or TRS cable.
The real answer when interfacing unbalanced and balanced gear is that they are not compatible. If you are being really picky anyway. But if you have to interface them then the negative lead should be either unconnected or terminated at the shield. This is opposed to the way a lot of people will teach you (the way I was taught actually) which is to tie the negative lead to ground.
Also don't get confused about TRS. For instance a headphone jack TRS is a stereo unbalanced signal but a line level or a microphone TRS jack will most likely be mono balanced. Like the headphone jack on the front of you reel to reel is stereo but the same type of jack on a soundcard will be mono.
So basically three options I recommend for you are
Akai>RCAto1/4"TS cable>soundcard
Akai>RCA(male)to1/4"TS(female) adapter>1/4" cable>soundcard
Akai>RCA cable>RCA(female) to1/4"TS(male) adapter>soundcard
Quasi balanced would be
Akai>RCA (negative lead unconnected) to 1/4" TRS> balanced soundcard