Unless you find the volume on your Shures is too low, I don't see any reason for using an amp/dac with the iPod. As long as you use lossless music, the amp/dac is not going to fill any void, becuase nothing's missing in the first place.
I use the SE425 with my iPod, and they can get pretty loud at 75% volume. I've never gone past 60%. The 535s are an even easier load to drive.
As for the sound changing in some way, its probably placebo, or the amps are distorting the sound in some manner, or changes in the listening environment (car, home, big room, small room, furniture, no furniture etc etc) can affect the experience.
The best way is to listen for yourself. If someone you know has an amp, ask him/her to give you a listening test, without you actually knowing if an amp is connected or not. That will be the ultimate test for your needs. If you feel the difference with the amp, by all means go ahead and buy one.