Apple did indeed "downgrade" their DAC chips within their iPod models, but the same model number iPods should have the same components. I believe the switch was made around in 2005-2007-ish perhaps (not 100% sure), so it really shouldn't be affecting you.
here are some links you can read about it:
if you want to use the newer iPods/iPhones that may have inferior dac chips, you can try to find an external dac/amp... but with apple's proprietary connection, it seems kinda expensive. more info here:
If you use a non-apple product w/ a standard mini-usb plug that can do usb audio out (ex: some android smart phones), it's pretty cheap & easy to get a portable external dac that will help your sound quality (if that is the issue that you are experiencing); such as the Hifimediy usb dac ($35-60: ES9023 sabre dac chip) -> can use an adapter to convert usb to mini-usb.
$30 HiFimeDIY Sabre Android DAC: http://hifimediy.com/index.php?route=product/product&path=62&product_id=124
$150 Fiios E18 is a dac/amp combo for android.
Also, there many portable amp/dac combos like the Fiio E7 $70, E10 for $75, E70k for $90, E17 for $130 vs. iBasso d-zero for $100 vs. audioquest dragonfly 1.0 $99 vs dragonfly 1.2 $150... etc...
Finally, you can also get a different dedicated audio player such as the Fiio X3 vs. iBasso DX50 with a built-in amp/WM8740 dac chip for $200.