Right now (if I understand correctly) you are plugging into the Guitar Port and going through USB into the mac. So the Guitar Port is acting as a direct box - taking a high impedance signal - and an AD converter. You are asking if it would help to put another direct box in front of the Guitar Port's direct box. Without knowing anything about the two boxes you mention or having used the Guitar Port, I'm going to guess, "no." Even assuming that those DIs are better than the DI in the Guitar Port (probably true - the Guitar Port does several things - they do one) I doubt you hear much of an improvement putting them in series.

Rather than spending on this right now you might save toward a better overall interface that would have a superior direct box/preamp/AD chain.

thanks! what would be a superior direct box/preamp/AD chain? would it be something in the 300 dollar range or 1000 dollar range?

I don't really know much about the options in the sub $300 range. Just a pure guess - but I think most will be about the same quality as the guitar port. Anything in that range is going to qualify as a cheap one or two channel interface. You already have one that is working pretty well for you.

If you want to make a bigger investment, the thing you really want to decide is what are your long term goals for playing and recording. If you want to do more than record guitar (e.g. vocal, keys, drums, acoustic instruments) that means a very different interface than someone who is interested in only one direct channel for guitar. There are all kinds of thread on interfaces on this board, but it really depends on what you want to do with them. If you want a good interface with a small number of channels, someone else will have to chime in with opinions. I have not really looked at that segment of the market.

thanks!