Just to let you know, there are only 2 portable products in existence that will bypass the ipod's internal DAC chip. Those are the ALO Solo and the HP-p1. The NuForce iDo MIGHT do it but im like 90% sure it is just an amp, like the portatube.
I'd go with the alo solo over the hp-p1. Those two products though will make the biggest difference. If you get an LOD cable for the ipod, you can bypass the ipod's volume control circuitry and internal opamp, send that pre-amped post-dac'd signal to a portable amp, and that will accomplish an audible increase in sound quality/signature. However, the portable DACs will make the biggest difference.
Thank You for your advice. I will definitely take a closer look at the ALO Solo. I just got my Shure se535 today and it sound pretty good but I know it can be better with the right portable amp.
I will keep you posted on which amp I decided to go with. Many Thanks.